Add a mechanism for making aliases of glyphs

Add a mechanism for making aliases of glyphs

This is done by making the new glyph contain only a reference to the
old one.  While it's possible to have multiple Unicode code points
reference the same glyph, that's rarely what you want because it's
useful to be able to map back from a glyph name to a code point, and
obviously that requires that the glyph have different names when
accessed through different code points.