Tree of source() calls

[moodymudskipper]

It's hard to make a tree of source() calls statically because they might be conditional and the path might be a variable or a call.

However we can source and build a tree of source calls as we go.

It could be displayed a bit like lobster::ast(), the last shown path is the last that was entered.

To do this we need to override source().

Each call to source updates a tree object on entry. We need a behaviour on exit too so we can know our depth in the tree.