Many of the instruction functions correctly match the size of immediate values, but they only accept unsigned immediates. This PR fixes the operand check functions for intermediate types to also accept the signed variants.
Fixes#181
It was pointed out #73 that Bytes() is a poor name for the size of the register in bytes. In idiomatic Go you would probably expect a Bytes() method to return []byte.
This diff changes the Bytes() to Size(). As a result the Size type also needed to be renamed, and Width seemed a reasonable choice.
Fixes#73
Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().
Closes#1
