Unify string representation across the codebase by switching SymbolTable and method name storage from a manually-managed `String` struct (char* buffer + length) to `ObjString*` pointers. This eliminates duplicate string allocations in the meta API's `metaGetModuleVariables`, reduces memory management overhead by leveraging the existing GC for symbol strings, and adds `wrenBlackenSymbolTable` to properly trace these GC-roots during collection. The change touches the compiler, debug dumper, VM runtime error formatting, and module variable initialization to use `->value` and `->length` fields directly from `ObjString`.
Michel Hermier
4e53cda68d