don't use Symbols for strings

string expressions by and large do not need the benefits a Symbol gives us,
instead they pollute the symbol table and cause unnecessary overhead for almost
all strings. the one place we can think of that benefits from them (attrpaths
with expressions) extracts the benefit in the parser, which we'll have to touch
anyway when changing ExprString to hold strings.

this gives a sizeable improvement on of 3-5% on all benchmarks we've run.

 before

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.844 s ±  0.045 s    [User: 6.750 s, System: 1.663 s]
    Range (min … max):    8.758 s …  8.922 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     367.4 ms ±   3.3 ms    [User: 332.3 ms, System: 35.2 ms]
    Range (min … max):   364.0 ms … 375.2 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.810 s ±  0.030 s    [User: 2.517 s, System: 0.225 s]
    Range (min … max):    2.742 s …  2.854 s    20 runs

 after

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.533 s ±  0.068 s    [User: 6.485 s, System: 1.642 s]
    Range (min … max):    8.404 s …  8.657 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     347.6 ms ±   3.1 ms    [User: 313.1 ms, System: 34.5 ms]
    Range (min … max):   343.3 ms … 354.6 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.709 s ±  0.032 s    [User: 2.414 s, System: 0.232 s]
    Range (min … max):    2.655 s …  2.788 s    20 runs

src/libexpr/nixexpr.hh

@@ -110,9 +110,9 @@ struct ExprFloat : Expr
 
 struct ExprString : Expr
 {
-    Symbol s;
+    string s;
     Value v;
-    ExprString(const Symbol & s) : s(s) { v.mkString(s); };
+    ExprString(std::string s) : s(std::move(s)) { v.mkString(this->s.data()); };
     COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
 };