patch applied (ghc): Add assertion for arity match (checks Trac #2844)

[Simon Marlow]

Simon Peyton Jones wrote:
> Mon Dec  8 09:32:41 PST 2008  simonpj at microsoft.com
>   * Add assertion for arity match (checks Trac #2844)
>   
>   The exported arity of a function must match the arity for the
>   STG function.  Trac #2844 was a pretty obscure manifestation of
>   the failure of this invariant. This patch doesn't cure the bug;
>   rather it adds an assertion to CoreToStg to check the invariant
>   so we should get an earlier and less obscure warning if this
>   fails in future.

Hooray for the testsuite and validate: out of 2000+ tests, there was 
exactly 1 (ONE) that demonstrated this bug, and it was in a test for the 
System.Random library of all things.  This serves as a reminder to all of 
us: any test failure should be treated as suspicious, even if it doesn't 
look at all related to something you changed, and this is exactly why we 
have to keep validate clean.  I am now a fully paid-up validate convert!

Cheers,
	Simon