Fix assorted issues in convert_to_scalar().

 Fix assorted issues in convert_to_scalar().
 
 If convert_to_scalar is passed a pair of datatypes it can't cope with,
 its former behavior was just to elog(ERROR).  While this is OK so far as
 the core code is concerned, there's extension code that would like to use
 scalarltsel/scalargtsel/etc as selectivity estimators for operators that
 work on non-core datatypes, and this behavior is a show-stopper for that
 use-case.  If we simply allow convert_to_scalar to return FALSE instead of
 outright failing, then the main logic of scalarltsel/scalargtsel will work
 fine for any operator that behaves like a scalar inequality comparison.
 The lack of conversion capability will mean that we can't estimate to
 better than histogram-bin-width precision, since the code will effectively
 assume that the comparison constant falls at the middle of its bin.  But
 that's still a lot better than nothing.  (Someday we should provide a way
 for extension code to supply a custom version of convert_to_scalar, but
 today is not that day.)
 
 While poking at this issue, we noted that the existing code for handling
 type bytea in convert_to_scalar is several bricks shy of a load.
 It assumes without checking that if the comparison value is type bytea,
 the bounds values are too; in the worst case this could lead to a crash.
 It also fails to detoast the input values, so that the comparison result is
 complete garbage if any input is toasted out-of-line, compressed, or even
 just short-header.  I'm not sure how often such cases actually occur ---
 the bounds values, at least, are probably safe since they are elements of
 an array and hence can't be toasted.  But that doesn't make this code OK.
 
 Back-patch to all supported branches, partly because author requested that,
 but mostly because of the bytea bugs.  The change in API for the exposed
 routine convert_network_to_scalar() is theoretically a back-patch hazard,
 but it seems pretty unlikely that any third-party code is calling that
 function directly.
 
 Tomas Vondra, with some adjustments by me
 
 Discussion: https://postgr.es/m/b68441b6-d18f-13ab-b43b-9a72188a4e02@2ndquadrant.com