add some keywords to the crate. this is a first for yaxpeax

instead of trying to shoehorn in `adr reg, label` syntax like the manual
requests, it's much easier to just describe these as `{add, sub} reg,
pc, offset` and potentially rewrite `pc, offset` as an `adr reg, label`
if a higher-level tool has that kind of information available.

add in some simd tests for future neon decoding. these tests are drawn
from capstone and may need some subsequent cleanup