Disregard stack, use register machine with infinite registers

[vendethiel]

compileFn returns

data VarIdx = Local Int | Temp Int | NoRes
compileFn :: ... -> (VarIdx, [Instructions])

instead of the whole stack thing, when encountering a variable decl, introduce a new Local, and when compiling an expression, a new Temp.
That probably means a State that keeps:

• Local mapping Map String Local (local name => var idx).
  Without shadowing, otherwise need to find a different way)
• The last Temp seen (and a function newTemp that increments the one in the State and returns it).