Fix #256 - renameID bug plus too few IDs allocatable for remapping

Created by: nthykier

This PR solves #256 (closed), which is really two issues combined:

1. There was a bug in renameID where if two IDs were swapped (AA <=> a), then all references would end up in one of the IDs (rather than correctly split between the two).
2. scour would only allocate 26 characters (a-z) for IDs. This branch bumps it to 52 (a-zA-Z), which solves the size issue in #256 (closed). Though in theory it can be increased even further (e.g. 0-9 can be used from character 2 and later in the ID) - this is left as future work.

Closes: #256 (closed)