Derive Copy + Clone for MaxpTable and CmapSubtable

[fschutt]

Also it would be great if lookup_glyph_index was on the owned::CmapSubtable - together with conversion function to convert from a CmapSubtable<'a> to an owned::CmapSubtable and back - this way users can cache the CmapSubtable if needed.

I'm trying to create a struct that "owns" the cmap subtable, but if I create an OwnedCmapSubtable then looking up glyphs isn't available anymore

[fschutt]

Also, please make the fields of SequentialMapGroup public

[fschutt]

See #40

[wezm]

Hmm right. The owned variant primarily exists to support writing fonts as part of subsetting. All of our glyph handling code goes via the Font type — if you're not already using it I'd suggest taking a look as it's intended to be the primary shaping interface. It handles several caches but does not yet have a full glyph cache.