sim_core: reworked register macros and descriptor lines

reworked register macros and instruction descriptor lines to more
 closely follow avr data sheet insruction (and avr instruction wiki)
 references. register macro usage (with few exceptions) have been worked
 throughout the core to be more uniform and consistent.  most notably d
 is used nearly uniform throught the avr instruction set references,
 prior the core intermixed the usage of the opcode registers d and r
 notation.

	modified:   sim/sim_core.c