System.Private.CoreLib support

[JoshuaWierenga]

This has the same purpose as #15 and describes important missing features from the full corelib that rather than build up from system features are based on existing features in corelib.

• [ ] Span. Requires Span.cs, ReadOnlySpan.cs, MemoryMarshal.cs and more complete versions of Unsafe.cs and Type.cs if not more.
• [ ] Nullable.
• [ ] Intrinsics/Vectors.
• [ ] System.Math.
• [ ] TimeZone. TimeZone.cs? TimeZoneInfo.cs?
• [ ] Tuple. Tuple.cs and ValueTuple.cs?
• [ ] Improved Primitive support
• [ ] Collections: List, Array, IEnumerable and Foreach, ...
• [ ] Exceptions

[JoshuaWierenga]

While GC is way off, https://github.com/kevingosse/ManagedDotnetGC might help.