Revert "Add debug tag to the GpuArray struct to help identify usage problems."

This reverts commit 50e20d53c44d7591896ff8badb37c472ad61b988.

This was a misguided attempt at making debugging easier by changing the ABI with a compile definition, which is a really bad idea.  The idea behind this commit may come back at some point in another form.
Use different kernel name for elemwise.
Add numpy c-api macro/fct to help make the conversion faster.
remove some fct that we don't want to provide from this interface.

Also Arnaud raised that in C, we can't call an fct as an initializer.