fixed slight bug in robot-field-read-register, in register namespace
removed passing of attributes from robot to init-register function. don't need them now.
ATTN: commits now contain internal info. Changed register system to
mostly an interface.

Removed :val field from all registers except storage registers & INDEX.

Some of the other registers were already quite specialized, but
the ones that weren't -- the ones that relied on :val fields in the
register -- are now just interfaces to attributes stored on the top
level of the robot.