perf(ff): parse bytes in 2^64 chunks; drop alloc in BE path

[radik878]

Description

Replace per-byte accumulation in from_le_bytes_mod_order with 8-byte chunking using a precomputed 2^64 window. This reduces field multiplications by ~8x on long inputs while preserving semantics and constant-time field operations. Also rework from_be_bytes_mod_order to avoid reversing into a new Vec by streaming big-endian input in 2^64 chunks.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Pratyush]

Thank you for the PR! Could you add tests that show the behaviour is preserved? E.g. specifically in test-templates?

[radik878]

Thank you for the PR! Could you add tests that show the behaviour is preserved? E.g. specifically in test-templates?

Did it!