rename SymbolIdx -> Symbol, Symbol -> SymbolStr

after #6218 `Symbol` no longer confers a uniqueness invariant on the
string it wraps, it is now possible to create multiple symbols that
compare equal but whose string contents have different addresses. this
guarantee is now only provided by `SymbolIdx`, leaving `Symbol` only as
a string wrapper that knows about the intricacies of how symbols need to
be formatted for output.

this change renames `SymbolIdx` to `Symbol` to restore the previous
semantics of `Symbol` to that name. we also keep the wrapper type and
rename it to `SymbolStr` instead of returning plain strings from lookups
into the symbol table because symbols are formatted for output in many
places. theoretically we do not need `SymbolStr`, only a function that
formats a string for output as a symbol, but having to wrap every symbol
that appears in a message into eg `formatSymbol()` is error-prone and
inconvient.

src/libexpr/eval.hh

@@ -78,7 +78,7 @@ public:
 
     static inline std::string derivationNixPath = "//builtin/derivation.nix";
 
-    const SymbolIdx sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
+    const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
@@ -87,7 +87,7 @@ public:
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
         sPrefix;
-    SymbolIdx sDerivationNix;
+    Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */
@@ -269,14 +269,14 @@ public:
     [[gnu::noinline, gnu::noreturn]]
     void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3) const;
     [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx p1, const char * s, const SymbolIdx sym, const PosIdx p2) const;
+    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const PosIdx pos, const char * s) const;
     [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const SymbolIdx s2) const;
+    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-        const ExprLambda & fun, const SymbolIdx s2) const;
+        const ExprLambda & fun, const Symbol s2) const;
     [[gnu::noinline, gnu::noreturn]]
     void throwTypeError(const char * s, const Value & v) const;
     [[gnu::noinline, gnu::noreturn]]
@@ -392,7 +392,7 @@ public:
     inline Value * allocValue();
     inline Env & allocEnv(size_t size);
 
-    Value * allocAttr(Value & vAttrs, const SymbolIdx & name);
+    Value * allocAttr(Value & vAttrs, const Symbol & name);
     Value * allocAttr(Value & vAttrs, std::string_view name);
 
     Bindings * allocBindings(size_t capacity);