Make euclid build without libm

[nical]

Currently euclid can build in a no-std environment only if the libm feature is enabled. As a result, downstream crates that want to support no-std have to either add a libm feature or force the libm dependency even in the default std configuration.
There are users of euclid that don't need the float math functionality that is provided by std/libm, For example etagere and guillotiere that mostly use the types in their public API and some simple rectangle functionality that doesn't require libm.

This PR aims to address that by sprinkling #[cfg(any(feature = "std", feature = "libm"))] until euclid builds with --no-default-features.
It isn't super pretty, but it's simple to review and can be easily followed up with a patch that consolidates all of these #[cfg] into a single per type in front of an impl block for all of the functions that need libm/std.

[nicoburns]

The arbitrary and malloc_size_of impl's seem to have diappeared from vector.rs. Otherwise this looks good to me.

[nical]

Oh boy. The rebase clobbered a ton of unrelated changes.