Keywords to reserve for Rust2018 epoch

[scottmcm]

fallible or try, from

Bikeshed: Rename `catch` blocks to `fallible` blocks

I was contemplating catch blocks again, and musing on the feedback that we shouldn’t use exception terminology since we don’t have exceptions, and the fact that catch blocks in RFC #243 don’t really work like catch blocks in languages like Java anyway (nor like handle in SML). What if we renamed them fallible blocks? For example: type SizeHint = (usize, Option<usize>); pub fn combine(a: SizeHint, b: SizeHint) -> SizeHint { let low = a.0.saturating_add(b.0); let high = fallible { a.1?.…