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

mostly confusion of pre/post-increment, operand widths, immediate
widths, things of that nature

16-bit instructions only, for now

Operands was an enum covering all forms of operands and data for each
operand for all instructions. this is hard to iterate, and hard to work
with when a single operand is the one of interest, so it's now replaced
by an array of `Operand` enum instances like other architectures.

in the course of this change, several forms of decoding are broken,
while adding support for some earlier-unsupported multiplies and
  instructions like msr and clz

also clearly note which document the comments mentioning page
numbers/figures is referencing