Avoiding allocating and reallocating slices when getting a list of strong checksum matches
Fixes & tests for returning more than one index result
All reference blocks are passed on by the comparer if there is duplication

Duplicates should be resolved by the merger to ensure that there is some local source
for every possible reference block if available.
Adding more tests to the merger. Starting integration of LLRB as replacement to map

Added tests to check for correct behaviour in various cases where blocks are
duplicated in the reference or local file. Also added a small test to try out
the LLRB library (which has an incorrect example). The usecase will be clear in
following commits.
Merger now detects blocks within existing spans.
Comparison of a file to itself now works consistently almost as expected