reduce copies during drv parsing

many paths need not be heap-allocated, and derivation env name/valye
pairs can be moved into the map.

before:

Benchmark 1: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      6.883 s ±  0.016 s    [User: 5.250 s, System: 1.424 s]
  Range (min … max):    6.860 s …  6.905 s    10 runs

after:

Benchmark 1: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      6.868 s ±  0.027 s    [User: 5.194 s, System: 1.466 s]
  Range (min … max):    6.828 s …  6.913 s    10 runs

src/libstore/derivations.cc

@@ -233,10 +233,10 @@ static void validatePath(std::string_view s) {
         throw FormatError("bad path '%1%' in derivation", s);
 }
 
-static Path parsePath(ParseContext & str)
+static BackedStringView parsePath(ParseContext & str)
 {
-    auto s = parseString(str).toOwned();
-    validatePath(s);
+    auto s = parseString(str);
+    validatePath(*s);
     return s;
 }
 
@@ -260,7 +260,7 @@ static StringSet parseStrings(ParseContext & str, bool arePaths)
     StringSet res;
     expect(str, "[");
     while (!endOfList(str))
-        res.insert(arePaths ? parsePath(str) : parseString(str).toOwned());
+        res.insert((arePaths ? parsePath(str) : parseString(str)).toOwned());
     return res;
 }
 
@@ -432,9 +432,9 @@ Derivation parseDerivation(
     expect(str, ",[");
     while (!endOfList(str)) {
         expect(str, "(");
-        Path drvPath = parsePath(str);
+        auto drvPath = parsePath(str);
         expect(str, ",");
-        drv.inputDrvs.map.insert_or_assign(store.parseStorePath(drvPath), parseDerivedPathMapNode(store, str, version));
+        drv.inputDrvs.map.insert_or_assign(store.parseStorePath(*drvPath), parseDerivedPathMapNode(store, str, version));
         expect(str, ")");
     }
 
@@ -453,7 +453,7 @@ Derivation parseDerivation(
         expect(str, "("); auto name = parseString(str).toOwned();
         expect(str, ","); auto value = parseString(str).toOwned();
         expect(str, ")");
-        drv.env[name] = value;
+        drv.env.insert_or_assign(std::move(name), std::move(value));
     }
 
     expect(str, ")");