Reduced string interning in Shader generation.

At least for the RenderMan case, our parameters only go one level deep (no structs), so we don't need to construct any new InternedStrings at all. This gives a very mild (~1.5%) reduction in runtime fora benchmark scene generation test.
Merge pull request #1024 from johnhaddon/shaderStringInterning

Reduced string interning in Shader generation.