Replace robust-predicates

[hannobraun]

robust-predicates contains C code, which has caused issues with cross-compilation in the past (see #1278, for example). This pull request replaces it as a dependency, with two components:

• robust, a pure-Rust library created from the same C code. It only provides 2D predicates, unfortunately, meaning it isn't a full replacement.
• fj_math::robust, a new module in fj-math with transpiled code from the same C source, which provides the missing 3D predicate.

Adding all this code isn't ideal, but since this comes from an old and stable C library (public domain, no less), I don't think it will be much of a problem. Still, it would be better to contribute the missing predicate to robust instead. I've decided against that for now, as I don't have any tests for this code, beyond the indirect coverage of our own test suite.

I will monitor how the new code works out, and might re-evaluate this decision at a later point.