See Note [exprArity invariant] in CoreUtils.  In code generated by Happy
I was seeing this after TidyPgm and CorePrep

	f :: Any
	f {arity 1} = id `cast` unsafe-co

So f claimed to have arity 1 (because exprArity looked inside), but
did not have any top-level lambdas (because its type is Any).  

This triggered a slightly-obscure ASSERT failure in CoreToStg

This patch 
	- makes exprArity trim the arity if the type is not a function
	- adds a stronger ASSERT in TidyPgm

It's not the only way to solve this problem (see Note [exprArity invariant])
but it's enough for now.