Attempt to map U. F. O. ascent and descent values to native values better and handle failure to do so better.
Merge pull request #1654 from fontforge/issue_1645

Improve handling of U. F. O. ascent and descent values.
Merge branch 'master' into ufo_foreign_data_1
Switch arrays in U. F. O. so that they map to the PyList type.
Fix a leak.