Provide implementation for `lastBitSet`.

Fix serious bug in division and remainder logic for `BigInt` numbers.…

… Provide support for consistent bit counting and leading/trailing zero bit counts.

Use ContiguousArray instead of Array to improve performance.

Use SPM API Version 4.

Bug fix: bump up Swift major version.

Support operations reporting overflow in Rational. This functionality…

… was lost with the Swift 4 numeric API changes.

Fix typos.

Prepare new release.

Support converting `Double` values to `BigInt` values. Fix `toPowerOf`.

Update .gitignore file.