Procedural macro implementation for assert-struct.

This crate provides the procedural macro implementation for the assert-struct crate. Users should use the main assert-struct crate which re-exports this macro.

Architecture Overview

The macro transformation happens in three phases:

1. Parse (parse.rs): Tokenize the macro input into a Pattern AST
2. Expand (expand.rs): Transform patterns into assertion code
3. Execute: Generated code runs the actual assertions

Key Design Decisions

• Pattern enum: Unified abstraction for all pattern types (struct, tuple, slice, etc.)
• Explicit patterns: Require explicit operators (e.g., Some(== my_var) not Some(my_var))
• Dual-path optimization: String literal regexes compile at expansion time
• Native Rust syntax: Use match expressions for ranges, slices, and enums

See the main assert-struct crate for documentation and examples.