HexToDec: tighten up trigger to hex number only.

This was interfering with the HexToASCII goodie.  We will assume that
with other context they want something besides the conversion.

It should still be possible to add any explicit triggers into this
Goodie if so are desired. The tests indicate no such intention;
assuming this is fine for now.
HexToDec: move the guarding into the trigger.

I don't know why this didn't occur to me, but thankfully it did to
@mintsoft!
HexToDec: use more appropriate answer_type.
Merge pull request #643 from mwmiller/hex_triggers

HexToDec: tighten up trigger to hex number only.