Add RawPadding trait to block-padding for padding blocks of arbitrary size

[czinn]

Closes #860

The existing Padding trait is automatically implemented using raw_pad and raw_unpad from RawPadding.

Because all implementations are inlined, code generation for users of Padding is the same as before. I checked this by defining some functions (see below) to monomorphize the Padding trait for an instance of Padding and comparing the generated assembly (using e.g. cargo asm block_padding::concrete_pad --build-type release) before and after this change. I observed no differences.

pub fn concrete_pad(block: &mut Block<generic_array::typenum::U128>, pos: usize) {
    Iso7816::pad(block, pos);
}

pub fn concrete_unpad(block: &mut Block<generic_array::typenum::U128>) -> Result<&[u8], UnpadError> {
    Iso7816::unpad(block)
}

I'm not attached to RawPadding as a name, something like ArbitraryPadding could also work. I think Padding and BlockPadding for (what is currently called) RawPadding and Padding would be better, but it wouldn't be worth the breaking interface change.

[newpavlov]

Thank you! I will publish this crate in the current form, but I probably will redesign the traits in v0.4.