Add facilities for debugging/visualising the typechecker

[brendanzab]

Currently it's hard to get much insight into what the type checker is doing when it is checking a term, even with a debugger. This is a barrier to entry for helping new developers gain understand how the Pikelet type checking algorithm works. This is important if we want more developers to be able to confidently contribute fixes and features to the type checker. It could also help us fix bugs and spot opportunities for optimizations.

A debugger would output some directed graph showing how a Pikelet term is checked. We could potentially use Graphvis, or perhaps HTML for this. Console logging is another option.