v4.0.0

More changes ported from my other project: Numberick.

• #147 Custom index methods
• #146 Shift methods rework
• #145 30% faster decoding
• #144 Galaxy-brain radix exponentiation
• #143 Back to front encoding is fastest
• #142 Future-proof core integer dividingFullWidthReportingOverflow(_:)
• #141 Use xctestplan(s)
• #140 Improved string decoding
• #139 Add uninitialized(_:)