Create MVMJitCode structure

This is a single structure that contains the function pointer
as well as the size of the fragment, the number of locals, and
the 'magic bytecode' which is now constant.
Use special register for CU

CU is actually used more often than the frame pointer
or the envionment (because the latter is often a lookup
'higher up' in the frame hierarchy). So it makes more
sense to store it directly rather than look it up through
two or more indirections.