New primitive Low* operation: ignore

[msprotz]

Companion PR in https://github.com/FStarLang/karamel/pull/273

You can override the definition of KRML_HOST_IGNORE with something more suitable if need be by invoking krml with -add-early-include '"mydefs.h"', then defining KRML_HOST_IGNORE in mydefs.h before the default definition in include/krml/internal/target.h kicks in.

Please confirm the function signature is good.