Toward common subexpression elimination in subscripts: generate C (and related languages) via an AST

Introduce kernel option to ignore boostable_into field in iname duplication

I have some kernels where I run into trouble with the boostable_into branch of get_iname_duplication_options.
It would be nice to be able to fully disable that branch. I think a kernel option is the correct way to implement such switch.

See merge request !3

as talked about.