math: Allow "x" for multiplication

[faho]

It's always a bit annoying that * requires quoting.

So we allow "x" as an alternative, only it needs to be followed by
whitespace to distinguish it from "0x" hexadecimal notation.

See e.g. issue #5650, #5324.

The obvious counterpoints here are that people will still try * (i.e. how many people do we save from having to ask?), and that it's a tad unclean. It might even lead to confusion if people try to spell hexadecimal numbers with spaces between them. The counter-counterpoints are that all we can do is try, it's not that unclean and that hex numbers are rarely used, and we don't support arbitrary spaces inside of numbers anyway (python supports _, but AFAICT it requires writing your own strtod, which isn't my idea of a fun weekend).

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[mqudsi]

fwiw, the type of people to use hex in a shell are probably the kind of people that would be caught dead writing 0x deadbeef with that space in there. +1 from me!

_{^{... however, I have always wondered if I'd still be able to look myself in the mirror if we special-cased * for math.}}

There. I said it.

[ridiculousfish]

LGTM