I always found calls to TcCanonical.xCtEvidence hard to grok; and I
found that we only had two left. This patch eliminates them, along
with xCtEvidence, its accompanying comments, and the auxiliary
XEvTerm type.

The two remaining calls were these:

 * One was in newSCWorkFromFlavored, where we'd already done
   case-splitting for given/wanted/derived.  So inlining the xCtEvidence
   made the code simpler, clearer, and faster.

 * The other was in canTuple; here all of xCtEvidence's functionality
   was needed, but inlining again made a net gain in code size and
   clarity.