k256: use group crate WnafBase/WnafScalar for GLV vartime mul

Replace custom wNAF implementation (wnaf_128, build_odd_multiples,
WnafSlot, wnaf_ladder) with the group crate's WnafBase/WnafScalar
types and WnafBase::multiscalar_mul_array.

A new WnafScalar::from_le_bytes constructor accepts short (128-bit)
GLV half-scalars, producing ~half the wNAF digits and ~half the
doublings in the evaluation loop. multiscalar_mul_array avoids the
two collect() heap allocations of the iterator-based multiscalar_mul.

Depends on RustCrypto/group#15 for the group
crate changes (wnaf_table size fix, from_le_bytes, multiscalar_mul_array,
pre-sized Vec allocations).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 42Pupusas

Cargo.lock

@@ -28,4 +28,4 @@ primefield = { path = "primefield" }
 primeorder = { path = "primeorder" }
 
 elliptic-curve = { git = "http://github.com/RustCrypto/traits.git" }
-rustcrypto-group = { git = "https://github.com/RustCrypto/group" }
+rustcrypto-group = { git = "https://github.com/42Pupusas/group.git", branch = "wnaf-optimizations" }