Encapsulate and slightly optimize string contexts

These steps are done (originally in order, but I squashed it as the end
result is still pretty small, and the churn in the code comments was a
bit annoying to keep straight).

1. Create proper struct type for string contexts on the heap

   This will make it easier to change this type in the future.

2. Make `Value::StringWithContext` iterable

   This make some for loops a lot more terse.

3. Encapsulate `Value::StringWithContext::Context::elems`

   It turns out the iterators we just exposed are sufficient.

4. Make `StringWithContext::Context` length-prefixed instead

   Rather than having a null pointer at the end, have a `size_t` at the
   beginning. This is the exact same size (note that null pointer is
   longer than null byte) and thus takes no more space!

Also, see the new TODO on naming. The thing we already so-named is a
builder type for string contexts, not the on-heap type. The
`fromBuilder` static method reflects what the names ought to be too.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>

src/libexpr/eval-cache.cc

@@ -136,17 +136,19 @@ struct AttrDb
         });
     }
 
-    AttrId setString(AttrKey key, std::string_view s, const char ** context = nullptr)
+    AttrId setString(AttrKey key, std::string_view s, const Value::StringWithContext::Context * context = nullptr)
     {
         return doSQLite([&]() {
             auto state(_state->lock());
 
             if (context) {
                 std::string ctx;
-                for (const char ** p = context; *p; ++p) {
-                    if (p != context)
+                bool first = true;
+                for (auto * elem : *context) {
+                    if (!first)
                         ctx.push_back(' ');
-                    ctx.append(*p);
+                    ctx.append(elem);
+                    first = false;
                 }
                 state->insertAttributeWithContext.use()(key.first)(symbols[key.second])(AttrType::String) (s) (ctx)
                     .exec();