Imports of pextgen command

PathSynopsis
bytesPackage bytes implements functions for the manipulation of byte slices.
flagPackage flag implements command-line flag parsing.
fmtPackage fmt implements formatted I/O with functions analogous to C's printf and scanf.
go/formatPackage format implements standard formatting of Go source.
math/bitsPackage bits implements bit counting and manipulation functions for the predeclared unsigned integer types.
osPackage os provides a platform-independent interface to operating system functionality.