Complete LetExp node.
Complete IfExp node.
Complete AssignExp node.
Complete BreakExp node.

I think this is incomplete, but apparantly TC-4 has more to say on the matter.
If does at least compile cleanly!
Complete RecordExp node.
Complete RecordTy node.
Complete SeqExp node.
Complete default visitors for thus far completed nodes.
Update completed stuff in README.
Merge branch 'master' of github.com:charlesturner7c5/tiger