movzlq fix
Optimisation - allow stack deref to remain on stack

We swap STACK for STACK_SAVE to say a value is on the stack. This lets
us delay dereference until later, e.g.
addl -8(%rbp), %eax
instead of
movl -8(%rbp), %ebx
addl %ebx, %eax
Allow STACK_SAVE as a store target in impl_store()
Moved stack save logic to -fremain-stack (arg parsing pending)
x86 float conversion can use the stack
Fixed v_to() for const values
Integral addition can use the stack
Removed v_to_reg_const (v_to() instead)
STACK_SAVE variant for memory - example
Added float cast test