OSL built-in displace() and bump() were documented but never implemented

[lgritz]

Add them now. Also, an additional variety that takes a vector offset in a non-common space.

There's no good reason why these were never implemented. I think that people just used the P += ...; N = calculatenormal(...) idiom directly. But they were always documented in the OSL Language Spec, so I see no reason not to add them now.