Replace separate strlen(buffer) calls with the length returned directly by sprintf in both
num_toString and range_toString native functions. This avoids an unnecessary pass over the
formatted string buffer, improving performance for these hot-path number and range conversions.
Refactor string_contains, string_count, string_eqeq, string_bangeq, and string_subscript to use the precomputed ObjString->length field instead of calling strlen() on the C string. This avoids O(n) length calculations on every string comparison and count operation.
Also update markString memory accounting to use the stored length plus null terminator instead of recalculating with strlen().
Add a `length` field to the `ObjString` struct in `wren_value.h` and initialize it in `wrenNewUninitializedString()` in `wren_value.c`. This stores the string's byte count excluding the null terminator, enabling O(1) length retrieval while preserving null-terminated C string compatibility.
Remove separate heap allocation for string text by storing it directly in the ObjString struct using a C99 flexible array member. This eliminates the need for an extra pointer indirection and a separate memory allocation call in wrenNewUninitializedString, while also simplifying the cleanup logic in wrenFreeObj by removing the explicit deallocation of the string's value buffer.
Evan Shaw