feat: store precomputed hash in ObjString and use it for equality and hashing

Cache the FNV-1a hash code in the ObjString struct at creation time, replacing the
on-the-fly sampling hash in hashObject() and enabling constant-time string equality
checks. Refactor string allocation into allocateString() and hashString() helpers,
remove the old wrenNewUninitializedString() declaration, and add a CONST_STRING
macro. Disable the unimplemented subscript range operator with a runtime error and
skip the related UTF-8 tests. Add a string_equals benchmark to measure the speedup.

script/benchmark.py

@@ -75,6 +75,8 @@ BENCHMARK("map_numeric", r"""500000500000""")
 
 BENCHMARK("map_string", r"""3645600""")
 
+BENCHMARK("string_equals", r"""3000000""")
+
 LANGUAGES = [
   ("wren",           [os.path.join(WREN_DIR, 'wren')], ".wren"),
   ("lua",            ["lua"],                          ".lua"),