Imports of standard command

PathSynopsis
github.com/mmcloughlin/avo/buildPackage build provides an assembly-like interface for incremental building of avo Files.
github.com/mmcloughlin/avo/irPackage ir provides the intermediate representation of avo programs.
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.
osPackage os provides a platform-independent interface to operating system functionality.
stringsPackage strings implements simple functions to manipulate UTF-8 encoded strings.