Static check noalloc

[gretay-js]

This is the first part of #707 :

• only allocation check
• only backend, command-line flag -check-alloc and cmx file format changes
• no attributes
• all raises are allowed (raise with trace is not treated as allocating)

[mshinwell]

@xclerc is going to review the parts under the backend directory, I'll review the remainder.

[xclerc]

Just made a first pass (would like to read
Checkmach again if we are not in a hurry.)

Two global questions:

• are we sure we want the checks to always be
  hard errors, or are there workflows where
  treating them more like warnings would make
  sense?
• is it difficult to adds some tests? (I don't know
  ocamltest enough to be sure, but it feels like
  we could check the basics through simple
  compiler invocations.)

[gretay-js]

@xclerc I've addressed the comments.

[gretay-js]

• are we sure we want the checks to always be
  hard errors, or are there workflows where
  treating them more like warnings would make
  sense?

There are some internal consistency checks that fail with fatal_error, but you are probably referring to the error raise if function doesn't confirm to its noalloc annotations (that are implemented in a separate PR). First of all, nothing is annotated to start with, and the noalloc check is off by default in user code (on by default in the compiler distribution). When annotations are added, if a change in a callee causes an "@Assert noalloc" annotation to be violated, then either the callee needs to be fixed, or annotated with "@Assume noalloc". If this is not enough, we can add some way of turning them into warnings. I first wanted to make it a warning not an error, so that can be turned into a hard error using -warn-error, but Warnings is not used in th backend and it didn't seem right to add one.