New features:
- i64_to_field(): Convert i64 (including i64::MIN) to field elements
- bigint_to_field(): Convert signed BigInt with automatic modular reduction

These functions enable safe conversion of large integer coefficients
to field elements, addressing the limitation where field.int_hom().map()
only accepts i32 values.

Changes:
- Add i64_to_field() with proper handling of i64::MIN edge case
- Add bigint_to_field() for arbitrary-precision signed integers
- Comprehensive test coverage for all edge cases
- Export new functions in public API