Move templates for static C code to the "C" folder

[chrisrichardson]

Move much of the code generation which is C-specific to the "C" folder, and dispenses with CNodes for it.

• Mostly static data for form, dofmap, finite_element, which does not need to use any language nodes
• Removes unneeded language items from CNodes (if, else, etc.)

This is part of a bigger transition to providing wider language support.