Produce ref assemblies from command-line and msbuild

[0x53A]

It would be nice if fsc could produce reference assemblies (see dotnet/roslyn#17558) and participate in the corresponding msbuild feature: dotnet/msbuild#1986.

This can potentially reduce (re)build times by a magnitude.

Deterministic builds (#2954) are probably a prerequisite for this.

Here is the Roslyn spec: https://github.com/dotnet/roslyn/blob/master/docs/features/refout.md

[abelbraaksma]

Some good discussion points are in the referenced issue, in particular this comment, which summarizes several views from other threads: #8642 (comment)