Implement The Compressor

[steveblackburn]

The Compressor is more or less the state of the art in compacting compressor GCs. It would be excellent to have an implementation in MMTk.

References:

• ACM digital library: https://dl.acm.org/doi/10.1145/1133255.1134023
• Author's copy: http://www.cs.technion.ac.il/~erez/Papers/compressor-pldi.pdf

[k-sareen]

A single-threaded in-place Compressor was merged in #1340.

[qinsoon]

Be aware that the current implementation has some assumptions about the runtime. And it does not work for runtimes that cannot satisfy the assumptions. See #1340 (comment) and #1353.