feat: make BigUint::serialized_size allocation-free

[Snezhkko]

Previously, BigUint::serialized_size in serialize/src/impls/int_like.rs computed the size by calling to_bytes_le() and delegating to Vec’s size calculation. This performed a temporary Vec allocation solely to obtain the byte length, which is unnecessary and introduces avoidable overhead. The change replaces this with an allocation-free calculation based on BigUint::bits(), preserving the exact on-wire format including the zero edge case. For zero values, to_bytes_le() returns a single byte [0], so we return 8 + 1 for the length-prefixed encoding. For non-zero values, the payload length equals ceil(bits/8), so we compute 8 + ((bits + 7) / 8) directly. The behavior of serialization remains unchanged; only the size computation path is optimized to avoid allocation.