hx::Val for __Run dynamic calling

@hughsando mentioned it might be useful to update the dynamic calling to use `hx::Val` instead of `Dynamic` to avoid some boxing. Seems like it should be pretty straight forward, given the signature is `Dynamic __Run(const Array& inArgs)` I think it would only be possible on the return type.

Start by locating the __Run dynamic-calling entry point with the signature Dynamic __Run(const Array& inArgs). Compare the current Dynamic return path with hx::Val and determine whether the requested change is limited to the return type. Done means the dynamic call uses hx::Val as intended without unnecessary boxing.

Written by the indexing model from the issue text.