Clarify definition of "input positions" in lifetime elision RFC.

Explicitly note that lifetimes from the `impl` (and `trait`/`struct`) are not considered "input positions" for the purposes of expanded `fn` definitions.

Added a collection of examples illustrating this.

Drive-by: Addressed a review comment from @chris-morgan [here](https://github.com/rust-lang/rfcs/pull/141#discussion_r14227369).

Fix #165.

arg, typo here, "`impl` header `impl`" probably should be just "`impl` header".  (I was originally going to spell out which components of the `impl` header I am referring to here, but then decided mid-stream not to bother.)