WIP Multiple font size / line height spans

[tigregalis]

Current status: Almost done. Working, with some fixes for the editing required.

Fixes #64

Implement specifying multiple font sizes and/or line heights.

Media8mute.mp4

TODO:

• fix scrolling
• implement enum LineHeight
• cache line_heights()
• retire Metrics
• rebase
• migrate example editor-orbclient
  • fix bug empty buffer does not render cursor
  • fix bug empty buffer has default Attrs
  • support rescaling font sizes on demand, perhaps via mutable access to spans of Attrs
• migrate example editor-test
• migrate example editor-libcosmic (NOTE: does not compile on Windows with pinned version of iced (incompatible API))
• migrate example terminal (NOTE: does not compile on Windows with termion, see Update terminal example using colored #129)
• migrate tests
• migrate benches
• migrate vi feature
• fix editor action Vertical
• fix some editor actions not redrawing
• tidy up
• docs and PR write-up

Future work:

• Split Attrs: color_opt, metadata, font_size, line_height
• Buffer should have a default Attrs
• Convenience method for updating all spans of Attrs in a Buffer
• Handle start-of-line Attrs more carefully

Questions:

• Should the Hash impl of Attrs include line height?
• Should Metrics be restored as a property of Attrs, with definition Metrics { font_size: f32, line_height: LineHeight }?

[nicoburns]

Linking my PR that continues this PR: tigregalis#1