Replaces the old LimEx NFA engines, which were specialised for model
size and number of shifts, with a new set of engines that can handle a
variable number of shifts.
Previously, we disallowed the merging of all Rose prefixes in block mode
where the literal sets are not identical.
This change allows merging if the prefix graphs to be merged are very
small, as a small performance improvement for cases with lots of tiny
prefixes.
This check is deliberately conservative: graphs must have some common
vertices, and the result of the merge must not give up any
accelerability.
