Recently we moved checkForInterruptSlow to .cpp, which prevents inlining.

We suspect it might causes some performance issues.

We should move it back to .h to restore performance.

The problem is that we can't include curop.h in expression_context.h, because curop.h uses expression_context.h (indirectly).

Most probably we will move only CurOp part to .cpp and put it in checkForInterruptVerySlow().