Where clauses that do not involve parameter types should be rejected

[nikomatsakis]

RFC 135 specifies that where clauses that involve no generic parameters, such as where int : Eq, are illegal. We should add a test that checks that a suitable error is reported. Note that the type parameter may appear outside the self type, however, so something like where int : Foo<Option<X>> should be fine.

[jroesch]

As a note a test like this should fail once we make the change: https://github.com/jroesch/rust/blob/generalized-where-clause-parser/src/test/run-pass/where-clauses-not-parameter.rs.