Fix missing initialisers for delitmusified local variables

[MattWindsor91]

When we delitmusify in vars-as-parameters mode, the initialisers of local variables are lost. This is less than optimal.

There are myriad ways to fix this, with advantages and disadvantages:

• convert to assignment statements (easy to implement, probably makes most sense, but generates double assignments);
• insert mappings into the Litmus test init block (single assignments and fairly clean, but overlapping variable names cause issues);
• insert a new mapping into the aux var_map block (unambiguous, but there are now two points of truth for variable initial values: this and the Litmus header)

I think I'll convert to assignments for now, and maybe add other solutions as flags later on.

[MattWindsor91]

(NB: I fixed this with the third option for now, as it turned out perversely to be easy-ish to implement; the first option may become a flag later)