Function ghash_mul 

pub fn ghash_mul<U: Underlier128bLanes>(x: U, y: U) -> U

Multiply two GHASH field elements using software implementation.

Method described at:

• https://www.bearssl.org/constanttime.html#ghash-for-gcm
• https://crypto.stackexchange.com/questions/66448/how-does-bearssls-gcm-modular-reduction-work/66462#66462

This code does not conform to the bit-endianness requirements of the GCM specification, but is a valid GHASH field multiplication with the modified representation.