Reduce transient allocations when stringifying hash keys

[OskarEichler]

Summary

Keep the existing key snapshot and Hash#[] lookups while filling the result directly instead of allocating a pair array per entry.

Reproduction

require 'representable/json/hash'
require 'representable/hash/allow_symbols'
input={a: 1, 'a'=>2, b: false}
p Representable::Hash::Conversion.stringify_keys(input)
# {'a'=>2, 'b'=>false}; input, collision behavior and ordering remain unchanged.

Verification

• Existing master suite on this isolated branch: 545 tests / 701 assertions, zero failures/errors, three existing skips on Ruby 3.2.11. Ruby 4.0.6 retains the same eight legacy inspection-format failures, zero errors and three skips. Existing tests/expectations are unchanged.
• 1500 focused checks on Ruby 3.2.11/4.0.6 for this change. XML first-render uses a separate fresh-process check.
• Combined release-based verification with other focused fixes and attributed Use Setter in OverwriteOnNil if a :setter is defined #177/Hash value of null breaks parsing #264/Update MultiJSON usage to address deprecations #273 source backports: 544 tests / 700 assertions pass on Ruby 3.2.11, plus 4,410 focused checks and 36 boundary checks. Package retains 104 paths and unchanged compared metadata. All 36 runtime files compile on both Rubies; no newly introduced default Lint offenses.

Isolated YJIT measurement: three alternating process pairs, each with five batches of 1,000 conversions of a 100-entry hash. Median time 0.009058→0.007898s; allocations 203,000→102,000. An earlier each_with_object variant was slower and rejected. No production throughput claim.

Breaking changes / limitations

No intended breaking change. Key snapshots, custom Hash#[] behavior, collisions, ordering and value references remain. No recursive conversion or newer Hash#transform_keys API is introduced.

No new/modified checked-in tests. Ruby 2.4–3.1, JRuby, Windows and Linux were not executed locally. Ruby >=2.4 and runtime dependency ranges remain unchanged. External test tooling supplies a legacy MiniTest alias and ruby-prof 1.7.x; no assertions were changed. No production verification is claimed.