Address #158: Filename encoding for Darwin
Enable utf-8 encoding of filenames for Darwin.
See #158 for the complicated rules.
This is incomplete. We support utf-8, but we currently don't apply the rules about using conjoining jamos nor the rules about the exceptions for canonically decomposed characters.
The code to support converting Hangul to conjoining jamos is included as well as the changes needed to exclude certain codepoints from decomposition. But the code to call these functions is not implmeented. This is pending resolution of other issues raised in the comments.