For DECODE-UNIVERSAL-TIME, recursing once per 400 years is fine for
small negative numbers but it can blow the stack if you accidentally
feed it a large magnitude negative number. Rather than recurse until
positive, figure out the necessary offsets from the start and go.

For ENCODE-UNIVERSAL-TIME, make the corresponding change, and drop the
CL guessing behavior surrounding years 0–99.

* declare it to be under the MIT license
* include a bunch of macros and utilities from QUUX (have to be moved in their own package!)