Use BitLength for all bit-denoted lengths

[briansmith]

See also #345, 346, #347.

Without the compiler supporting const fns, we are limited in the contexts in which we can use BitLength. @samscott89 noted that we may be able to make BitLength a newtype struct BitLength(usize), which might be able to let us use it in more contexts before const fns are available. That sounds good to me, but we should add some TODO comments about reversing that once we have const fns.

Similarly, we have code like this for converting from bits to bytes:

                public_key_len: 1 + (2 * (($bits + 7) / 8)),
                elem_and_scalar_len: ($bits + 7) / 8,

We could define a bits_as_bytes_rounded_up! macro that would allow us to write:

                public_key_len: 1 + (2 * bits_as_bytes_rounded_up!($bits)),
                elem_and_scalar_len: bits_as_bytes_rounded_up!($bits),

Again, if we do that then we should have a TODO to remove the macro when const fns are available.

[briansmith]

Searching for "8 ", " 8", "+ 7", and "/ 8" is a good way to find relevant code.

[briansmith]

Also, it's a good idea to update STYLE.md to mention the conventions for using BitLength, assuming usize-typed lengths are in bytes, using the _len/_LEN suffix for byte-delineated lengths, and using the _bits/_BITS suffix for lengths delineated in bits.

[pietro]

Some work done on the branch mentioned on #346. If you're ok with that I'll go after the "/ 8" left in the codebase.

[briansmith]

In order to do this completely we'd need to handle non-usize types, especually u64 for SHA-256/SHA-384/SHA-512, and unsigned for GFp_AES_set_encrypt_key`.