[outlineCompiler] disallow multiple glyphs mapping to same unicode
[outlineCompiler] disallow multiple glyphs mapping to same unicode

Take the first glyph found in the font.glyphOrder, and print a warning
in case other glyphs map to the same unicode value.

Fixes #213

moyogo/ufo2ft,jamesgk/ufo2fdk,googlefonts/ufo2ft,jamesgk/ufo2ft,googlei18n/ufo2ft