Rename `external "runtime"` to `intrinsic`

[WilsonGramer]

Rename external "runtime" "foo" to intrinsic "foo" so that the namespace provided to external is now a path to a file:

instance (Add Number Number Number) : a b -> intrinsic "add-numbers" a b

This enables the ability to select which file the external function belongs to:

do-something-in-js : external "path/to/script" "do-something" 1 2 3