Add garbage collector to implementations table and descriptions

It seems there is quite a bit of variance here in the implementations. Each implementation page should have a description of that implementations approach to garbage collection, with links to official documentation if possible.

Any reputable comparisons to other implementations in speed/other metrics should also be included.

Current sbcl notes:

• Baker/cheney two space collector. Occasionally-conservative generational collector.
• May want to mention with-pinned-object.
• Proposal to remove cheney collector on table.

Source: IRC
Manual is sparse but here: http://www.sbcl.org/manual/#Garbage-Collection

Current CCL notes:

• Precise, generational, compacting garbage collector.
• Excellent docs: http://ccl.clozure.com/docs/ccl.html#understanding-and-configuring-the-garbage-collector

Current CLISP notes:

• Sounds like a precise compacting generational collector?
• Not sure, see http://clisp.org/impnotes.html

Current lispworks notes:

• Also a generational collector, haven't read enough to determine whether it is also precise/compacting.
• Good docs here http://www.lispworks.com/documentation/lw70/LW/html/lw-77.htm

Current allegro CL notes:

• Global parallel (woot!) garbage collection.
• Most advanced garbage collection in current implementations?
• http://franz.com/support/documentation/10.0/doc/release-notes.htm#acl-new