Improve parser and interpreter performance

[danielgtaylor]

Summary

• Reduced allocations in the lexer, parser, and interpreter hot paths.
• Added focused benchmarks to track common expression cases and internal helpers.
• Optimized slice-length handling and primitive equality checks to avoid unnecessary work.
• Kept behavior stable with coverage updates in the existing tests.

Testing

• go test ./...
• Benchmarks run locally for the common slow and cached expression cases, plus new internal microbenchmarks.

[Copilot]

Pull request overview

This PR focuses on improving performance in the expression engine by reducing allocations and adding benchmarks to track key hot paths across lexing, parsing, and evaluation.

Changes:

• Reduced allocations by reusing lexer token storage and avoiding interface indirection in parser construction.
• Added fast paths for slice .length evaluation and primitive equality checks.
• Introduced internal benchmarks to measure common expression/utility performance.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
typecheck.go	Reuses shared option parsing to avoid repeated option scanning.
parser.go	Avoids interface calls when using the concrete lexer; supports generic lexers as fallback.
lexer.go	Reworks token allocation/caching and avoids per-token rune counting in hot paths.
interpreter.go	Adds bound-normalization helpers and a fast path for slice .length.
functions.go	Adds a fast-path switch to avoid reflection when resolving non-lazy values.
expr.go	Centralizes option parsing and constructs lexer/parser/interpreter/typechecker with fewer allocations.
conversions.go	Adds fast paths for deep equality of numbers/strings/bools (including lazy funcs).
benchmark_test.go	Adds focused internal benchmarks for lexer, lazy-value resolution, and equality checks.

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