This uses SAW's new `llvm_points_to_bitfield` command (introduced in
GaloisInc/saw-script#1539) to simplify specifications that involve the
bitfields in `s2n_config` and `s2n_connection`. In particular, there is no
longer any need to manually compute the size of the bitfields or the indices of
the fields within the bitfields, as these details are handled automatically
by `llvm_points_to_bitfield`. The positions of the fields within bitfields are
also no longer important.

This addresses a review comment in
aws#3079 (comment).

These comments are no longer relevant now that the SAW proofs use the
`llvm_bitfield_points_to` command, which is insensitive to the order of fields
within a bitfield.