sd-bus: use terms from the D-Bus Specification a bit more

sd-bus: use terms from the D-Bus Specification a bit more

D-Bus' type hierarchy as described in the spec is:

\- basic
   \- fixed type (u, i, etc.)
   \- string-like type (s, o, g)
\- container

Someone seems to have referred to basic types as "simple types" at
some point, but that term isn't defined in the D-Bus Specification,
and seems redundant.

So far I haven't renamed functions that use "trivial" in their names
to mean "fixed type", to avoid confusion about whether a struct of
constant length, like (iu), is a fixed type. The answer is that it is
fixed-length, but is not a "fixed type", so I can see that something
like bus_type_is_fixed() might be ambiguous.