Truffle: fill out the rest of the set_trace_func functionality

[chrisseaton]

Truffle has an exceptional implementation of set_trace_func in terms of performance, but only enough features have been added to support writing up a paper and we need to fill out the rest of it.

I think we need to add trace events for entering and leaving methods, to differentiate between Ruby and 'C' methods, and maybe some other stuff.

[chrisseaton]

Closing because we'd like anyone who wants to work on Truffle to speak to us first.