Proposal - disallow overloading of opIn

[Jarrett Billingsley]

By that, I mean just the "regular" form of opIn, and only allow overloading 
of opIn_r.

Why?  Well, 'in' doesn't make sense if you reverse it; that is, "array in 4" 
makes no sense while "4 in array" does.

Just from a cursory glance at opover.c, this should not be hard to 
implement - just have InExp::opId() return NULL.  It doesn't look like 
anything depends on the result of opId() being non-NULL.