Replace Math folder with equivalent System.Numerics classes

[CallumDev]

This aids interoperability with code written to target .NET Standard

[AndresTraks]

Yeah, it's probably a good idea by now.
System.Numerics was introduced in .NET 4.6 and I wanted to keep compatibility with .NET 4.0, which is not so relevant now.
I'll consider it.

[AndresTraks]

There are currently no double precision System.Numerics types, so the double precision branch would have to keep using BulletSharp.Math.

[MikePelton]

Double-precision support for System.Numerics expected in .NET 6, perhaps late 2021.

[epicabsol]

I have done a port to System.Numerics (excluding demos and 64-bit support) in #84.