Support new CLI type: half type / float16

[abelbraaksma]

Half type

.NET 5 introduces a new primitive, we should probably add it to our list of primitive your aliases. My suggestion: half and/or float16.

See:

• https://devblogs.microsoft.com/dotnet/introducing-the-half-type/.
• implementation details: Half: An IEEE 754 compliant float16 type dotnet/runtime#37630
• proposal: API Proposal: Add System.Numerics.Half 16 bit floating point number conforming to IEEE 754:2008 binary16 dotnet/runtime#936

Pros and Cons

The advantages of making this adjustment to F# are parity with primitive types in dotnet 5.

The disadvantages of making this adjustment to F# are none.

Extra information

Estimated cost (XS, S, M, L, XL, XXL): XS

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

For Readers

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[abelbraaksma]

Comment from @pblasucci from original thread:

Just a note to say: if/when this work gets done, the work being tracked in #495 should also be extended (it'll be easy-enough).

Also, I'm quite happy with either (or both) half and float16. 👍

[cartermp]

@dsyme I went ahead and approved this in principle

[abelbraaksma]

@cartermp RFC PR is ready for review: fsharp/fslang-design#499 (some open questions arose while writing it).

RFC discussion is here: fsharp/fslang-design#500.