Disallow (most) reserved/keywords as lifetime names

[huonw]

e.g.

fn a<'type, 'struct, 'fn>() {}

is currently valid, but shouldn't be. (Note that 'static is, and should remain, a valid lifetime.)

[huonw]

Note that this will only be possible after a snapshot, since the snapshot compiler currently requires lifetimes called 'self.

(Tagged easy although asking for some guidance in IRC may be useful since the parser isn't exactly easy to navigate.)

[thestinger]

Why do we need to disallow this?

[huonw]

Consistency, maybe? (@nikomatsakis suggested it.)

[nikomatsakis]

@thestinger motivation was both consistency and to allow for possible future special uses. In particular I think 'self could possibly be put to better use in the future.