The Context.Label method and LABEL global function did not agree. Also
breaks the convention I'd like to set that capitalized functions must
agree with existing Go assembly syntax.
To help avoid a conflict with `avo.Label`, attributes were moved to
their own package.
Fixes#35
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
Previously we updated the set of live in registers before live out. This
was extremely inefficient, since on each pass through live in depends on
live out.
We also change to processing the instructions in reverse order, which is
more likely to be efficient, although we should replace this with
topological sort order soon.
