Initial Port from calends-php

The original version of this library was built in PHP, but as that's not a very portable implementation, and can't be used in other languages, this version was written to replace the other as the primary implementation. The library it creates can be wrapped into a PHP extension, obsoleting the original implementation (partly by being considerably faster), as well as being usable by other languages using their own "external library" access mechanisms.

Completed functionality is indicated in the README, but to summarize:

• Range and precision
• Date parsing, date formatting, and offset parsing for the following calendar systems:
  • TAI64 (internal values, out to "roctoseconds")
  • Unix timestamps
  • Gregorian
• Instants and spans
• Interfaces for adding calendar systems

Other features are still pending implementation, which is part of why this library is considered to be at a prerelease version.