create clean ld instruction which handles the basic ld/store functionality with zero conditionals

Conflicts:
	lib/gbrb/cpu/z80.rb
define separate class to handle indirect ld targets

Conflicts:
	lib/gbrb/cpu/z80.rb
separate class to handle indirect ld destinations
load direct values into combined registers
load direct values into single registers
load and increment hl
load and decrement hl
increment and decrement ld with indirect destination
load immediate value into memory address stored in hl
load from and to memory addresses stored in bc and de registers
some more types of ld instruction
two more ld instructions
remove ld_complex and simplify ldh