[sgen] Free contiguous blocks on 32 bit archs.
[sgen] Try allocating M&S blocks in smaller contingents in case of failure.

This might happen when the address space fragments.  If it happens due to
OOM, we're probably lost anyway.
[sgen] While iterating over empty block array, compact it.
[sgen] Statistics for 32 bit freeing.
Merge branch 'feature-free-32'