Enable entanglement detection by default (prepping for v0.3 release)

[shwestrick]

This patch enables entanglement detection by default, which has nearly zero overhead (see #154) for most of our benchmarks.

[shwestrick]

After merging this, I'd like to do a v0.3 release.

Putting together a list of major changes since v0.2:

• (Offband hierarchical heap objects, separate union-find nodes, and epoch-based reclamation #131) more efficient hierarchical heaps and union-find heap lookup
• (Massive performance improvement: new chunk allocation system #135) Hoard-inspired block allocator
• (Parallelize the CC stack (forgotten set) #141) parallelization of "forgotten set" (snapshot reachability)
• (Multiple major updates #142) CC-chaining (major performance improvement for CGC)
• (Multiple major updates #142) down-pointer pinning
• (Upstream merge #153) upstream merge from MLton
• (Switch to work list loop for CC #147, Better space performance for CC work list #150, Fix CC work list space leak #160) CC work list
• (Multiple major updates #142, Enable/disable entanglement detection at compile-time in the runtime system #145, Fast entanglement detection based on entanglement candidates (suspects) #154) entanglement detection

Have I missed anything?