tmp/fmt: introduce a complete D-Bus type-system

Introduce the `fmt::dbus` module with a complete D-Bus type-system. This
adds the `element` module, as well as the `signature` module. Combined
they provide full parsers of D-Bus type strings with support for a range
of variants:

- `DVariant`: The classic D-Bus wire format as defined by the original
  dbus-specification, but including extensions up to the current
  version.

- `GVariant`: A complete rework of the wire-format, but type-system
  compatible with `DVariant`. It is mainly used by the GNOME project and
  glib.

- `Json`: A new `GVariant`-compatible version, but using JSON as
  encoding.

`dbus::Sig` is the main type of the type-system. It represents a D-Bus
Single Complete Type, but provides pre-formatted information on the full
type and all its sub-types.

`dbus::Cursor` is used to iterate over a type and its individual
elements. It is only needed when introspecting unknown types.

Signed-off-by: David Rheinsberg <david@readahead.eu>

Author: dvdhrm