[IMPL] Add core Vector3 functionality

[FiniteReality]

Fixes #100.

I've not vectorized these, and have instead only provided the software fallback, because I'm not sure I trust myself to correctly implement it. Plus, I believe there probably isn't much benefit to vectorizing these methods specifically, since they deal with small amounts of data - the overhead of shuffling data about would probably outweigh the performance benefits of vectorizing the code currently.

Maybe in the future we can add methods to take vectors in bulk to implement the vectorized versions of these algorithms.

Unit tests are in progress.

[FiniteReality]

I've currently omitted the single-parameter constructor since I cannot easily think of many scenarios where it would be clearer than Vector3.One * value.

[tannergooding]

I've currently omitted the single-parameter constructor since I cannot easily think of many scenarios where it would be clearer than Vector3.One * value.

It's also for efficiency in the implementation. Once it's vectorized, it will forward to the implementation that does a broadcast, rather than having some other implementation.