Don't permit object-like operations on ptypes
Thread checks in where it makes sense.
Test merge arguments too
Insert checks for merge arguments
Test path arguments
Per @mlucy, standardize error message & implementation
Bump reql_version_t

v1.16 introduces changes in operations on pseudotypes.
Only do checks for v1.16

This should address some of the sindex problems; at least we use the
old behavior.  Not sure how to proceed with warnings.
Remove switch and replace with statement.

The existence for the switch statement is some dimly remembered "we
might have more complicated logic some day" assesment.  We don't.
When we do have more complicated logic, we can put it back in, but
until that day it just confuses things.
Per @tryneus, improve backtrace targets.
Verify better error message with test.
Target right object
Merge branch 'gchpaco_2766' into next

Fixes #2766.  This isn't a good idea to put in 1.15, so it is only going to next.