Check class instances against their definitions.

[benl]

We don't currently check whether a type class instance is valid wrt its definition. Unsafe program sometimes make it through the core type checker and sometimes not.

T2-CheckInstance contains an area function that has a larger closure than the definition, but it is silently accepted. I've seen other programs like this be caught by the core type checker.

[lilac]

It looks like the current T2-CheckInstance? testcase should not be diagnosed by this check once ticket#63 is fixed.