libstore: Get rid of allocations in printString, allocate 2K bytes on the stack

Looking at perf:

   0.21 │       push   %rbp
   0.99 │       mov    %rsp,%rbp
        │       push   %r15
   0.25 │       push   %r14
        │       push   %r13
   0.49 │       push   %r12
   0.66 │       push   %rbx
   1.23 │       lea    -0x10000(%rsp),%r11
   0.23 │ 15:   sub    $0x1000,%rsp
   1.01 │       orq    $0x0,(%rsp)
  59.12 │       cmp    %r11,%rsp
   0.27 │     ↑ jne    15

Seems like 64K is too much to have on the stack for each invocation, considering
that only a minuscule number of allocations are actually larger than 4K.

There's actually no good reason this function should use so much stack space. Or
use small_string at all. Everything can be done in small chunks that don't require
any memory allocations and use up 2K bytes on the stack.

This patch also adds a microbenchmark for tracking the unparsing performance. Here
are the results for this change:

(Before)

BM_UnparseRealDerivationFile/hello         7275 ns         7247 ns        96093 bytes_per_second=232.136Mi/s
BM_UnparseRealDerivationFile/firefox      40538 ns        40376 ns        17327 bytes_per_second=378.534Mi/s

(After)

BM_UnparseRealDerivationFile/hello         3228 ns         3218 ns       215671 bytes_per_second=522.775Mi/s
BM_UnparseRealDerivationFile/firefox      39724 ns        39584 ns        17617 bytes_per_second=386.101Mi/s

This translates into nice evaluation performance improvements (compared to 18c3d2348f):

Benchmark 1: GC_INITIAL_HEAP_SIZE=8G old-nix/bin/nix-instantiate ../nixpkgs -A nixosTests.gnome --readonly-mode
  Time (mean ± σ):      3.111 s ±  0.021 s    [User: 2.513 s, System: 0.580 s]
  Range (min … max):    3.083 s …  3.143 s    10 runs

Benchmark 2: GC_INITIAL_HEAP_SIZE=8G result/bin/nix-instantiate ../nixpkgs -A nixosTests.gnome --readonly-mode
  Time (mean ± σ):      3.037 s ±  0.038 s    [User: 2.461 s, System: 0.558 s]
  Range (min … max):    2.960 s …  3.086 s    10 runs

src/libstore-tests/derivation-parser-bench.cc

@@ -28,6 +28,27 @@ static void BM_ParseRealDerivationFile(benchmark::State & state, const std::stri
     state.SetBytesProcessed(state.iterations() * content.size());
 }
 
+// Benchmark unparsing real derivation files
+static void BM_UnparseRealDerivationFile(benchmark::State & state, const std::string & filename)
+{
+    // Read the file once
+    std::ifstream file(filename);
+    std::stringstream buffer;
+    buffer << file.rdbuf();
+    std::string content = buffer.str();
+
+    auto store = openStore("dummy://");
+    ExperimentalFeatureSettings xpSettings;
+    auto drv = parseDerivation(*store, std::string(content), "test", xpSettings);
+
+    for (auto _ : state) {
+        auto unparsed = drv.unparse(*store, /*maskOutputs=*/false);
+        benchmark::DoNotOptimize(unparsed);
+        assert(unparsed.size() == content.size());
+    }
+    state.SetBytesProcessed(state.iterations() * content.size());
+}
+
 // Register benchmarks for actual test derivation files if they exist
 BENCHMARK_CAPTURE(
     BM_ParseRealDerivationFile,
@@ -37,3 +58,11 @@ BENCHMARK_CAPTURE(
     BM_ParseRealDerivationFile,
     firefox,
     getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");
+BENCHMARK_CAPTURE(
+    BM_UnparseRealDerivationFile,
+    hello,
+    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/hello.drv");
+BENCHMARK_CAPTURE(
+    BM_UnparseRealDerivationFile,
+    firefox,
+    getEnvNonEmpty("_NIX_TEST_UNIT_DATA").value_or(NIX_UNIT_TEST_DATA) + "/derivation/firefox.drv");

src/libstore/derivations.cc

@@ -498,28 +498,33 @@ Derivation parseDerivation(
  */
 static void printString(std::string & res, std::string_view s)
 {
-    boost::container::small_vector<char, 64 * 1024> buffer;
-    buffer.reserve(s.size() * 2 + 2);
-    char * buf = buffer.data();
-    char * p = buf;
-    *p++ = '"';
-    for (auto c : s)
-        if (c == '\"' || c == '\\') {
-            *p++ = '\\';
-            *p++ = c;
-        } else if (c == '\n') {
-            *p++ = '\\';
-            *p++ = 'n';
-        } else if (c == '\r') {
-            *p++ = '\\';
-            *p++ = 'r';
-        } else if (c == '\t') {
-            *p++ = '\\';
-            *p++ = 't';
-        } else
-            *p++ = c;
-    *p++ = '"';
-    res.append(buf, p - buf);
+    res.reserve(res.size() + s.size() * 2 + 2);
+    res += '"';
+    static constexpr auto chunkSize = 1024;
+    std::array<char, 2 * chunkSize + 2> buffer;
+    while (!s.empty()) {
+        auto chunk = s.substr(0, /*n=*/chunkSize);
+        s.remove_prefix(chunk.size());
+        char * buf = buffer.data();
+        char * p = buf;
+        for (auto c : chunk)
+            if (c == '\"' || c == '\\') {
+                *p++ = '\\';
+                *p++ = c;
+            } else if (c == '\n') {
+                *p++ = '\\';
+                *p++ = 'n';
+            } else if (c == '\r') {
+                *p++ = '\\';
+                *p++ = 'r';
+            } else if (c == '\t') {
+                *p++ = '\\';
+                *p++ = 't';
+            } else
+                *p++ = c;
+        res.append(buf, p - buf);
+    }
+    res += '"';
 }
 
 static void printUnquotedString(std::string & res, std::string_view s)