Speed up hash mode by interning element names directly

[Watson1978]

Ox.load(..., mode: :hash) creates a temporary Ruby String for every element name, associates its encoding, and immediately interns it. Repeated tags therefore allocate a new String even when the name is already interned.

Use rb_intern3() on the name bytes, preserving the parser's encoding and the ASCII-8BIT fallback for file input without an encoding. Keep interning after attribute processing, including attribute key modifier callbacks. This extends the element-name optimization in #474 to hash mode; attribute key handling is unchanged.

On Ruby 4.0.6 / arm64, parsing 40,000 records with five text fields per record:

	Before	After
Parse time	46.10 ms	41.75 ms
Allocated objects	800,006	560,005

This is a 9.4% reduction in parse time. Each process warmed up twice, then measured seven parses with GC.start before each measurement. These are the medians of four process-level medians, alternating the before/after execution order. All result digests matched. The unchanged hash_no_attrs control measured 28.27 ms before and 29.23 ms after, with identical allocation counts.

Input:

xml = '<rows>' + '<row id="1"><name>Alice</name><city>Tokyo</city><age>30</age><status>active</status><note>hello</note></row>' * 40_000 + '</rows>'
Ox.load(xml, mode: :hash)

Validation:

• Added tests for repeated elements with and without attributes, UTF-8/Shift_JIS/binary names, Symbol and String keys, XML-declared encoding, and the file-input binary fallback.
• Added coverage for encoded names passed to attribute and element key modifiers, including allocation/GC during the attribute callback.
• test/hash_load_test.rb: 19 tests, 109 assertions passed against both the original and updated implementation. Also passed with ASCII-8BIT configured as the default encoding before running the suite.
• Additional *_test.rb suite, using the Rake test task's exclusions: 344 tests, 7,044 assertions passed.
• test/tests.rb and test/sax/sax_test.rb passed.
• clang-format --dry-run --Werror ext/ox/hash_load.c and git diff --check passed.