GC refactor ahead

[Vasily Chekalkin]

chromatic wrote:
>> Is there any way we can verify this presumption?
> 
> Profile NQP-rx on Actions.pm with Callgrind and look at the callees of pmc_new.  
> Of 4,780,813 PMCs created, the top 10 are:
> 
> *	890,230 CallSignature
> *	851,059 CallSignatureReturns

This 2 are easy to merge. Just move attributes of CallSignatureReturns 
into CallSignature, VTABLE functions into src/call/args.c.

-- 
Bacek