perf: eliminate String clone during serialization

[fabracht]

Summary

• Eliminate unnecessary heap allocation when serializing String fields
• String fields now use references instead of cloning during try_to_be_bytes()
• Remove unused is_copy function from utils.rs

Changes

• Changed create_field_accessor semantics from needs_owned to is_copy parameter
• Copy types (primitives, arrays) get value copy: let field = self.field;
• Non-Copy types (String) get reference: let field = &self.field;
• Removed 69-line is_copy function and 37 lines of related tests (dead code)

Breaking Changes

None - API unchanged, only internal optimization

Testing

• All 280+ existing tests pass
• macro_test binary runs successfully with String demos