Font embedding in SVGs

This adds funtions to add the capability for embedding fonts in output SVG files. This builds on the changes in diagrams/diagrams-svg#38 and diagrams/SVGFonts#9. Wait for them to be merged before applying these changes.

* Switched to using the `State` monad to pass around the internal rendering state.
* The built in fonts can now also be referenced by their real font family names.applyapply
* The diagrams backend now support to switch between native font rendering and SVG font rendering.
* When using native font rendering used glyphs are collected for each used font.
* Added four new helper functions that embed fonts in output SVG data or files.
* Added test runners for the embedded font versions. All tests pass fine.