Add value type parameters RFC

[rust-highfive]

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Tracking issue for postponed PR #174

[milibopp]

This should probably be closed as #174 was essentially the same idea as #56 by merging #174 and #56 into #273. As the author of #56 I'll look into this.

[leoyvens]

We now have an accepted RFC for this, #2000. This issue should be closed.