lib: update qualified imports for LibTest theories

lib: Replace subseq->match abbreviation.

The subseq_abbreviation mechanism was a useful way of quoting some of a
definition or term, specialised to the case of left-associated sequences.

Lambda abstractions are now handled better.

The previous subseq mechanism required some generalisations. It is now replaced
by match_abbreviation, which is a more general approach.

The match mechanism picks a term, can select a matching subterm, and can
rewrite the selected term based on pattern matching also. The new mechanism
can cover all the cases of the previous one, as shown in examples.