Update to encoding_rs, rustler 0.37, add dirty schedulers

[jeffhuen]

Summary

This PR modernizes excoding while maintaining full backwards compatibility:

• Switch to encoding_rs - The same encoding library used by Firefox. Better performance, actively maintained, and WHATWG compliant
• Update dependencies - rustler 0.29 → 0.37, rustler_precompiled 0.5 → 0.8, Rust edition 2018 → 2021
• Add dirty schedulers - Operations on binaries >64KB automatically use dirty CPU schedulers to avoid blocking the BEAM
• Better error handling - All Rust .unwrap() calls replaced with proper error propagation (no more panics)

Issues Fixed

• Closes Error when compiling against OTP-26 #40 (OTP-26 compilation errors) - rustler 0.37 has OTP-26/27/28 support
• Closes Rust decode<'a> and encode<'a> should not panic, instead they should return {:error, ...} tuple to elixir #24 (Rust functions should not panic) - All error paths now return proper errors

New Functions

# Tuple-based error handling (new)
Excoding.safe_encode(string, encoding)  # => {:ok, binary} | {:error, :unknown_encoding}
Excoding.safe_decode(binary, encoding)  # => {:ok, string} | {:error, :unknown_encoding}

# Utilities (new)
Excoding.encoding_exists?("utf-8")       # => true
Excoding.canonical_name("latin1")        # => {:ok, "windows-1252"}
Excoding.list_encodings()                # => ["UTF-8", "Shift_JIS", ...]
Excoding.dirty_threshold()               # => 65536

Backwards Compatibility

• encode/2 and decode/2 work exactly as before (return raw value, raise on error)
• No breaking changes to existing API

Test Plan

• All existing tests pass
• Added tests for new functions
• Roundtrip tests for various encodings (UTF-8, Shift_JIS, EUC-KR, Windows-125x)
• mix format passes
• mix compile --warnings-as-errors passes
• cargo clippy passes

Happy to adjust anything based on feedback!

🤖 Generated with Claude Code

[jeffhuen]

Closing this PR—after further development, the changes became substantial enough (switching to the encoding_rs crate, new streaming API, different architecture) that it made more sense to publish as a separate package.

Available as encoding_rs on Hex for anyone interested. Thanks for the original work on excoding!