Fix asInt conversion and test across Int8-128 & UInt8-UInt128

[kdubb]

The current implementation of asInt<I:FixedWidthInteger>() is currently broken. Amongst other issues, it fails to convert BigDecimal(0) and BigDecimal(1).

I fixed the implementation by:

1. Fixing the check for fractions
   It now uses a rounded (via truncation) value and then uses a simple truncated != self comparison to reject fractions.
2. Added a asFixedWidthInteger() extension function to BInt
   Uses BInt.asMagnitudeBytes() and sign conversion to build an integer of the appropriate size.

BInt.asInt vs asFixedWidthInteger()

Adding the asFixedWidthInteger() is necessary because BInt only supports asInt() -> Int (non-generic and native size only). Obviously this causes issues with integer types that have a range that differs from Int.

asFixedWidthInteger() instead uses BInt.asMagnitudeBytes() to export the bytes and copies them to an integer of the appropriate size, after which it modifies the sign as appropriate. Ultimately this, or a similar generic function, should probably be transferred to the BInt package.

With the conversion from BInt to any FixedWidthInteger encapsulated in an extension function, and because I'm using asMagnitudeBytes() to do the byte export from BInt, I was able to reduce the # of operations to a single withExponent(0, .towardZero) plus the range checks.

Warning

An issue with conversion from Swift's Int128 to BigDecimal was discovered, which is fixed in #11. Until that PR is merged, some of the Int128 tests will fail.

Note

I used Swift Testing for the test suite for its native support of parameterized tests which made testing and debugging much easier.