Fixes #19

Checks if a tracing channel declaration is already included in the method or script body before inserting it.

A couple of notes:

1. My Rust is almost nonexistent. But, from a straight programming standpoint, I think that this approach shouldn't be too time consuming, as the channel declaration should be at the very top of the file and we shouldn't have to iterate over too many lines. But please correct me if this implementation is bad in Rust (or in general).
2. The tests are a bit weird, asserting against the same context. I thought about using two different contexts with the same channel name, but the tests work as-is. If it's not readable, or just is more preferable in general to do two contexts, I am happy to do so instead!