Run asmfmt suring linting and confirm git repository isn't dirty.
This introduces a developer tools dependency on asmfmt, but not a
runtime dependency.
Updates #8
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
