Add FromMeta for Vec<LitStr> & co

[Ten0]

This allows parsing meta attributes of the form #[foo(bar = ["a", "b", "c"])] to Vec<syn::LitStr> (and same for other Lits).

[TedDriggs]

Why are these impls needed - wouldn't these all be valid Expr, or ExprArray? I'm not opposed to merging this, but I do recall there being some issue with Vec in the past, and I'm therefore wary of piecemeal adding Vec impls.

Update: I think I remember the backstory now.

I avoided adding a impl<T: FromMeta> FromMeta for Vec<T> because different lists would have different manifestations in the AST. Adding these specific impls is useful because it lets someone take a list of homogenous literals, rather than having to accept an Expr and then check it using and_then

I am okay with merging this once the format errors are fixed.