creating convenience types for make_def, make_stmt, make_methods in MacResult

I was experimenting writing a plugin and noticed that there were convenience types for a few of the methods in the `MacResult` trait but not for all of them.  So I added some for `make_stmt`, `make_methods`, and `make_def`.  For the last one, I noticed that essentially the same struct existed in macro_rules, so I removed it.