Use buttons for the warning tree

This makes it easier for tree nodes to have their own active content.
Macro for performing output in an info buffer
Allow trees to have lazily-constructed children

This allows very large trees to be explored, and it also allows
potentially cyclic trees.
Lazily expand children on demand in tree
Greater flexibility to clients of info buffers
Increase the print depth in *idris-events*

This makes some debugging easier.
Simplify protocol for :who-calls, :calls-who
Merge pull request #113 from david-christiansen/whocalls

Ask Idris for which functions call which other ones