Profile-driven optimizations targeting the most expensive ormar functions.
Key changes:

- Cache alias<->field_name mappings per model class, using Rust
  build_reverse_alias_map for O(1) lookups (was O(n) linear scan,
  called 406K times in profiling)
- Cache (col_name, field_name) pairs to avoid repeated SA column
  iteration in own_table_columns and extract_prefixed_table_columns
- Use set instead of list for selected_columns membership checks
- Cache get_name(lower=True), extract_db_own_fields, ormar_fields_set,
  and ForeignKey constructors dict
- Use frozenset for RelationProxy method check

End-to-end benchmark improvements:
- iterate: 24-30% faster
- first: 26-36% faster
- get_all: 18-19% faster
- saving: 17-25% faster
- select_related: 12-17% faster

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>