Add BlockPool allocator for GCNode allocation

[llbit]

This replaces the use of BDWGC for allocating GCNodes. The BlockPool allocator allocates small objects in an arena, this speeds up allocation and pointer lookup for scalars.

[codecov-io]

Current coverage is 70.16% (diff: 87.37%)

Merging #106 into master will increase coverage by 0.08%

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- Misses        28634      28703    +69   
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[kmillar]

Please add your unit tests under tests/rho

[llbit]

@kmillar Thank you for all the comments so far! I added unit tests and addressed your comments in the latest commit.

I decided not to use boost::intrusive::slist yet, because I did not find a way to integrate it with the quarantine freelist. Do you have an idea how I might get around the Address Sanitizer poisoned freelist with intrusive list? The only idea I have is that the intrusive list part of the freelist could be kept unpoisoned.

[ArunChauhan]

Your code uses the "const" qualifier on methods (which is good), but it is not used consistently. I suggest going over your code once to mark all the const methods. It helps the compiler optimize the methods.

[llbit]

@kmillar I can rebase this into one commit if it helps. I avoided doing that so far so that the old comments would not disappear.