Improvements to emit.mlp organization

[kayceesrk]

• The preproc_fun function looks independent of the target processor, so eventually it should be moved to a target-independent module, e.g. Emitaux. But actually I'd prefer to refactor all the computations of various aspects of functions (whether they contain calls or not, initial stack size, maximal stack size, ...) to after Linearize and before Emit. (We had this discussion for poll point insertion already.)
• Stack checking is done at the beginning of the function (in Emit.fundecl) while the rest of the function prologue is emitted by the Lprologue instruction since this PR: Add Lprologue ocaml/ocaml#2055 , for reasons related to "debugger support". (Say no more.) I would prefer all the prologue code to be emitted in one place. This could mean reverting PR#2055 and removing the Lprologue instruction.

These arose from the asynchronous reviews #742 (comment), and are scheduled to be done after MVP merge.