perf: remove redundant allocations & computations from generated code

Summary

• fix: decode field name in error from utf8
• fix: use format_ident instead of trim_start_matches
• fix: context used almost always to ignore context
• perf: remove redundant allocations & computations from generated code

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files