Rework MethodSerializer (+ fallout)

Rework MethodSerializer (+ fallout)

The main aim of this patch is to avoid instanceof-determined behaviour
(between MethodDeclaration and MethodDefinition):
* move getThrowsExceptions() and getBody() to Method;
* make getVisibility() and getBody() Optional;
* method declarations can specify thrown exceptions (which are
  separated by commas, not spaces).

In addition, use Java 8 lambdas where appropriate in MethodSerializer
and a few places elsewhere, and use Modifier instead of String for
modifiers.

Some clean-up:
* drop inferrable type parameters;
* use new ArrayList<>() instead of Lists.newArrayList();
* use Collections.singletonList() where appropriate.

Change-Id: I9ec10bf64d366056afacce3fd4038ba92d3c4e6e
Signed-off-by: Stephen Kitt <skitt@redhat.com>