Make the parser’s ‘expected <foo>, found <bar>’ errors more accurate

As an example of what this changes, the following code:

    let x: [int ..4];

Currently spits out ‘expected `]`, found `..`’. However, a comma would also be
valid there, as would a number of other tokens. This change adjusts the parser
to produce more accurate errors, so that that example now produces ‘expected one
of `(`, `+`, `,`, `::`, or `]`, found `..`’.

src/test/compile-fail/removed-syntax-fixed-vec.rs

@@ -8,4 +8,4 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-type v = [int * 3]; //~ ERROR expected `]`, found `*`
+type v = [int * 3]; //~ ERROR expected one of `(`, `+`, `,`, `::`, or `]`, found `*`