Move the _apply versions of the function under CLinkerOp
and clarify/update documentation.
Add c_{init,support,cleanup}_code_struct to allow inserting stuf at the struct level.
Fix documentation updates after comments.
Add documentation for the _struct() c_code functions.
Fix critical problem in the __props__ implementation.

This breaks all ops that use it otherwise.
Add test for struct code.
Merge pull request #2061 from abergeron/struct_support

Struct support