Recording mutated closures

[acondolu]

Why

This PR is a prerequisite for implementing generational garbage collection (PR #455). In fact, generational GC requires to keep track of all the closures in the older generation that contain a pointer to a closure in the younger generation. This means that whenever a closure in the older generation is mutated, we need to record it in a set which is usually called remembered set or mut_list.

What

The types of the closures that can mutated at runtime are: Thread State Objects (TSO), STACK, synchronising variables (MVAR), mutable variables (MUT_VAR), arrays with pointers (MUT_ARR_PTRS, SMALL_MUT_ARR_PTRS), CAFs, and of course THUNKs. For each of these closure types, we need to ensure that the PrimOps functions and and the builtin ones manipulating them record correctly their mutation.
We provide stub recordMutable and recordMutableCap builtin methods, which will enqueue mutated closures in a mut_list in a future PR. Note that in all cases recordMutable is called only if the mutated closure is clean, and after setting it as dirty.

How

The main changes are in the Cmm.h file (restored the recordMutable Cmm macro) and in the Builtins.hs file, where various calls to a stub recordMutable builtin functions are added.

[acondolu]

Before merging, we should ensure that updateThunk (and probably also messageBlackHole, but I'm not very familiar with that that function does) call recordMutable when blackholing. The problem is that Blackhole.hs is imported in Builtins.hs, so we need some refactoring before, to avoid the cyclic dependency.