signatures: make param and optional param count IV

signatures: make param and optional param count IV

During the course of parsing end exection, these values get stored
as ints and UVs, then used as SSize_t.

Standardise on IVs instead. Technically they can never be negative, but
their final use is as indices into AVs, which is SSize_t, so it's
easier to standardise on a signed value throughout.