cmd: simple command for decoding instructions

uses rsc's instruction decoder, seems to work okay
cmd, decode: limited support for reading from regs.

this really shouldn't be done here, but instead at the
command-creation level, where it would be nice to also evaluate
simple expressions (i.e. "%rip + 1").

but this works, for now.
cmd: abstraction of where and minor cleanups.
cmd: new command to identify the basic block we are in.
cfg: kill dead code.