Use &'static str in ParseError::UnrecognizedEOF and ParseError::UnrecognizedToken

[Kmeakin]

Replace Vec<String> with Vec<&'static str> in ParseError::UnrecognizedEOF and ParseError::UnrecognizedToken. Since the terminal names are all known ahead of time, there is no need to store them on the heap.

[nikomatsakis]

Thanks for the PR. This is a reasonable change, but I'm also not sure how much I want to commit to knowing things up-front and not having dynamically generated names. Is there a strong motivation for this, or is it more like "general hygiene"?

[Kmeakin]

Just general hygenie: I'm allergic to unnecessary allocations. If you want to keep the ability to have dynamically generated names, we could use Cow<'static, str>

[nikomatsakis]

That makes sense, but then the types are more complicated, which also doesn't seem worth it to me yet.