Implement and test exp for Ratio

[jacg]

Close #290.

Please have a look if this is up to standard. If so, I'd be happy to add exp2, ln, log, log2 and log10 in a similar fashion.

[iliekturtles]

Go ahead and merge this impl block with the trig block and update the comments. Same for the tests below. rename mod inv_trig to mod float and include all of the functions.

[jacg]

Done.

I have also included exp_m1 and ln_1p which weren't in the original list.

log takes a second argument, besides self. I've given it the same type as self but I'm not sure whether we want to be more flexible. So I've added that one in a separate commit.

I've also added [must_use = ...] to the inverse-trig functions which were already there. Also in a separate commit, in case you don't want it. The must_use is there from the beginning for all the new functions that I added.

[iliekturtles]

Things look good after a quick skim so I kicked off the test workflows. I'll review in full detail later today.

For the [must_use = ...] attributes I do want to keep them and will add a new issue to review other existing methods.