Add the database family (DSUM, DPRODUCT, DCOUNT, DCOUNTA, DAVERAGE, DMAX,
DMIN, DGET, DSTDEV, DSTDEVP, DVAR, DVARP) and engineering functions
(number-base conversions, BITAND/BITOR/BITXOR/BITLSHIFT/BITRSHIFT with
web-safe 24-bit-half math, and CONVERT for common length/mass/time/
temperature units). Doc and README updated; also corrected stale spilling
notes in doc/functions.md.