Fix hash and equals contract and align the simulator with Java

[marcospassos]

Fixes correctness issues found by running this runtime and the Java reference over the same grammars and inputs and comparing the output.

Changes

• Hash and equals now agree for ATN configs, prediction contexts and bit sets. Values that are equal no longer land in different buckets, so sets and maps stop degrading into linear scans.
• MurmurHash matches Java bit for bit. It uses PHP's native murmur3a when available and falls back to a pure PHP implementation otherwise.
• ATNState::hashCode() returns the state number, as in Java and C#.
• The anonymous Equivalence class in ATNConfigSet is now a named class, so the set can be serialized.
• setTrace(false) clears the tracer, so isTrace() reports the right value. Enabling twice no longer attaches two listeners.
• The lexer decodes its input once instead of on every read, and the hot loops in both simulators avoid repeated method calls.

These also make the runtime faster, but the gain varies too much per grammar to quote a single figure here.

Verification

• antlr4 runtime-testsuite: 361 of 362 descriptors pass. The one failure comes from antlr4's PHP.stg, not from this repository.
• Parse trees, token streams and error output are byte identical to Java across 68 inputs from the grammars-v4 corpus.
• PHPStan at level max is clean.

Closes #33
Closes #36
Closes #12

Supersedes #28 and #34, which this replaces.

No tests in this PR. They come in a follow up.