Float16 (half-precision floating-point)

[stephentyrone]

Implement the IEEE-754 Float16 type, conforming to BinaryFloatingPoint. Use x86 and arm half<->float conversions, and armv8.2 half-precision arithmetic when available.

[dwaite]

Would having this type in swift-numerics vs stdlib hinder its use as a SIMD type?

[dwaite]

For reference:

swift evolution discussion
https://forums.swift.org/t/add-float16/19370

an implementation in progress (PR)
swiftlang/swift#21738

[KyNorthstar]

Can this be closed now?