Fix defmfun expansion of default optional/key values
For methods on marrays, all default values for optional and key arguments were wrongly coerced to the array element type. This doesn't affect any current definitions in GSLL, but FFT functions with the stride specified, it will wrongly change the default value to the element type. This has been fixed by changing #'actual-class-arglist so that it looks up the type intended to be passed to the C function and coerces to that if it is available. Otherwise the expansion leaves the default value alone.