Remove old fixed-length vector syntax

[bstrie]

Addresses #3664.

[catamorphism]

One question: do we want to use the self.obsolete() machinery here instead of just making the old syntax erroneous? (See calls to self.obsolete() in syntax::parse::parser for examples...)

[brson]

I would like to parse all obsolete syntax for 0.5 if we can (they should be minimal from here out). In this case it probably means taking those few bits of code removed from the parser and putting them into try_foo methods in obsolete.rs. This is probably a subject we should discuss as a group though.

[brson]

obsolete syntax tests are all in compile-fail/obsolete-syntax.rs

[brson]

I have a branch here that parses the old syntax still. Will merge shortly.

[brson]

Hm, this also takes out the fixed length string syntax, str/N, that I didn't even know still existed. Do we have an alternative syntax for strings or are we simply planning to not have them? I think the later (&static/str seems fine for all practical string usage).

The affected test case is compile-fail/estr-subtyping.

[brson]

OK. Pushed to incoming. Thanks!