Accept strings of the form "0x0p0", with no dot.

[sunfishcode]

GLIBC's and musl's printf with %a formats zero as "0x0p+0", with no dot, so
accept strings of that form.

[youknowone]

Hi, I am really sorry not to reply anything to this PR for so long time.
I am a new maintainer and I want to add this feature in future hexf. But recently we changed codebase to fit recent rust environment, so it needs rebase to go to be merged. If you are still interested in this PR, would you please rebase it to recent code base? I promise I will do proper further work in that case.
If you are not instersted in this PR anymore, just please let me know. That's totally understandable after many months since creating the PR.

[sunfishcode]

Thanks! I've now rebased this, and added a few more tests too.

[youknowone]

thank you!