util: Add ParseUInt32 and ParseUInt64
Add error and range-checking parsers for unsigned 32 and 64 bit numbers.
The 32-bit variant is required for parsing sequence numbers from the
command line in bitcoin-tx (see #8164 for discussion). I've thrown in
the 64-bit variant as a bonus, as I'm sure it will be needed at some
point.
Also adds tests, and updates developer-notes.md.