Replace CodepointOrGlyphId enum with IntoGlyphId trait.

Created by: jimblandy

This implements the change proposed in #89 (closed). I've tested it against the ten most-downloaded crates that depend on rusttype (listed below), and none of them required source code changes.

Lattice
conrod
font-atlas
gfx-glyph
ggez
imageproc
orbfont
piston2d-graphics --features=glyph_cache_rusttype
radiant-rs
waterfall