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 `..`’.