Imports of _asm command

PathSynopsis
github.com/mmcloughlin/avo/buildPackage build provides an assembly-like interface for incremental building of avo Files.
github.com/mmcloughlin/avo/operandPackage operand provides types for instruction operands.
github.com/mmcloughlin/avo/regPackage reg provides types for physical and virtual registers, and definitions of x86-64 register families.