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The first officially released version of macros persisted
macro def -> impl bindings across compilation runs using a neat trick.
The right-hand side of macro definition (which contains a reference to an impl)
was typechecked and then put verbatim into an annotation on macro definition.
This solution is very simple, but unfortunately it's also lacking. If we use it
then signatures of macro defs become transitively dependent on scala-reflect.jar
(because they refer to macro impls, and macro impls refer to
scala.reflect.macros.Context defined in scala-reflect.jar).
More details can be found in https://issues.scala-lang.org/browse/SI-5940.
Therefore we have to avoid putting macro impls into binding pickles and
come up with our own serialization format. Situation is further complicated
by the fact that it's not enough to just pickle impl's class and method names,
because macro expansion needs knowledge about the shape of impl's signature
(which we can't pickle). Hence we precompute necessary stuff (e.g. the layout
of type parameters) when compiling macro defs.
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