add trace_macro!

[eholk]

It'd be nice be able to see a trace of all the macro expansions. One way to do this would be to add a flag to the expansion context and control its value with another syntax extension.

[graydon]

It appears you added this immediately after opening the bug: 1e96099. Closing. Reopen if I misunderstand.

[eholk]

Oh yeah, I guess I forgot to close the bug. Thanks for closing it!