This reverts commit 6e909e38.

This reverts commit 06afe21c.

Only build through librustsyntax, which some tests do rely on.

I had to xfail one existing test case (class-implements-int) because,
I think, of the same bug described in #2272.

    enter_opt was handling the (*) case wrong and causing a bounds check
    failure. Fixed it (the test case is one of the extracted ones from
    the reference manual)

Closes #2220

Test case disabled until a memory-leak issue is resolved.

Closes #2255

Closes #2185

Figure the rest out in the parser

Use lookahead in the parser