ghash: add FieldElement

[makavity]

#355

[makavity]

@tarcieri tried to implement belt-dwp with that terms.
Unfortunately, i can't do it clearly without convertations like that:

#[inline(always)]
fn convert(x: u128) -> u128 {
    x.reverse_bits().swap_bytes()
}

#[inline(always)]
fn convert_block(block: &Block) -> Block {
    convert(u128::from_le_bytes((*block).into()))
        .to_le_bytes()
        .into()
}

Because STB spec works in different notation.
If it is okay - i'll correct my PR for belt-dwp.

[tarcieri]

Heh, I had a half-finished version of this locally too, but we can go with this.

Because STB spec works in different notation.

Wait, it uses GHASH, but in little endian? Weird. Unfortunate they didn't use POLYVAL if they wanted little endian.

[makavity]

Wait, it uses GHASH, but in little endian? Weird. Unfortunate they didn't use POLYVAL if they wanted little endian.

Yeah, unfortunately :(