Refactor ClosureComponent to contain a weight directly. 

Refactor ClosureComponent to contain a weight directly.  So a
w*closure tends to just be a single ClosureComponent, not a separate
MUL and COMPONENT. This simplifies things a bit and removes some
overhead from closure construction.

Note that it is an incompatible change for renderers!  Renderers should
#ifdef OSL_SUPPORTS_WEIGHTED_CLOSURE_COMPONENT and
adjust their behavior appropriately.