Draft segment interface.
Intial 'segment' code integrated into all glyphset types.

Changed how subset works so segment can use subset for random-access types.
Merge branch 'master' into segment
Changed parallel glyphs to use segment interface.
Removed 'implicit matrix'
Removed point-mem-mapped-list as it was unused.
mend
mend
All glyphsets implementing 'segment' protocol.  Rendering quad-tree glyph-parallel works.
Renaming segment protocol parts.
Misc fixes
Simplify types.