Replace the use of the internal_report structure (for reports from
engines, MPV etc) with the Rose program interpreter.
SOM processing was reworked to use a new som_operation structure that is
embedded in the appropriate instructions.
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.
