[refactor] Common-up in checkBody

[acl-cqc]

This arose trying to merge #41 with already-on-main #48.

Three commits....

• the first (pure refactor) just commons up the two makeBox calls. This requires a slightly awkward currying of checkConnectorsUsed in order to preserve exact behaviour (specifically, passing in show tm rather than show (WC fc tm) in the Clauses case).
• the second changes the Clauses case to show (WC fc tm) rather than show tm, paralleling the NoLhs case (but with a fake FC). However this doesn't actually change the output (changing that to "foo" breaks tests, but this change doesn't), so seems harmless.
• The third (again, pure refactor) tidies up by inlining now-singly-called checkConnectorsUsed