Generational garbage collector

[acondolu]

Context

The current asterius garbage collector is a copying collector, traversing all live heap objects whenever invoked. Performing a complete ("major") garbage collection each time is quite expensive, and usual runtime systems employ generational collectors, where objects in the heap are segregated according to their age; most times, only the youngest objects are garbage collected ("minor" collection).

Changes

This PR implements a simple generational GC, with two generations: gen 0 is the youngest and gen 1 the oldest. It requires inter-generational pointers (from gen 1 to gen 0) to be recorded, so to used them as roots during a minor collection. All young closures that survive a minor collection are promoted to gen 1 (i.e. generations are not divided in additional steps).

Special care has to be taken so to handle correctly "pinned" blocks: in the asterius runtime, the closures allocated in pinned blocks will not be moved by the garbage collector - this means that a different kind of promotion applies to pinned closures.

Related PRs

• Fix Bdescr Cmm macro #466 Fix Bdescr macro
• Clean dirty closures during scavenging #476 Cleaning dirty closures during scavenging
• Recording mutated closures #454 Recording mutated closures
• Basic generational machinery in HeapAlloc #484 Generations in HeapAlloc

Work in progress

There is mainly one feature missing: deciding when the collector should select a minor vs major collection. Probably need to fine tune the parameters and verify reduced running times of tests.