Implement builtins.path

[ryantrinkle]

This Builtin should be added here.

This doesn't have a .exp file; I think that means it just needs to evaluate without error.

Fixes eval-okay-path (remove it from newFailingTests list).

[jwiegley]

This could be challenging, since I think these "enriched paths" are supposed to be interchangeable with other paths.

[xplat]

nix actually handles this by figuring out the store path and using that as a plain path object. There are no actual 'enriched path objects' in nix libexpr.

[xplat]

Actually now that i check more closely, builtins.path returns a string, not a path!

[domenkozar]

Right:

nix-repl> :t builtins.path { name = "patchelf"; path = ./patchelf; }
a string with context