Added

- constructor for `Blob` from a mutable slice
- `Font::empty` as a simple way to construct the empty font
- support for serializing a `GlyphBuffer`'s contents
- `create_harfbuzz_rusttype_font`: a new way to create a font with Rusttype font
  funcs (the old `SetRustTypeFuncs` trait is deprecated)

Fixed

- lifetime of slice returned by `Blob::get_data` (could cause UB)

Changed

- The rustup feature is no longer enabled by default
- `SetRustTypeFuncs` is now deprecated in favor of
  `create_harfbuzz_rusttype_font`
- Internal representation of smart pointers (possibly more safe now)
- Many improvementes to documentation