Implement ival-pre-fabs

[obround]

Implements the ival-pre-fabs function to replace ival-fabs usages in places where we don't need to actually compute the absolute value of endpoints, just compare them. Uses ival-pre-fabs in ival-hypot! and ival-pow2. Decreases memory usage on rival from 236MiB to 233MiB (1.15% less).

Although ival-cosh is even, ival-pre-fabs doesn't work right out of the box, likely because of some issues with overflows-at. I'll try to fix it, but it seems like overflows-at isn't really tested when used in ival-cosh in the tests (if you remove overflows-at from ival-cosh, all the tests still pass), so I'm trying to be careful with what changes I make for ival-cosh.

[pavpanchekha]

Remaining nits are minor enough that go ahead and merge after you fix them, no need for another nightly.