Ignore mappings with duplicate generated locations.

This code works under the assumption that the same generated location
should never map to multiple source locations or names.
Add test for ignore duplicate mappings.
Ignore duplicate mappings.
Update test for ignore duplicate mappings.
added test sorting with duplicate generated mappings
Handle null 'mapping.original'.
Remove debug code.
Make 'strcmp' sort null equal with the empty string.
Factor out sort comparator, use it to replace conditional.
Make 'ignore duplicate mappings' test more straightforward.
Merge branch 'ignore-duplicate-mappings'
Bumping version to 0.1.22