Generate default no-arg constructors directly as bindings rather than as AST and then bindings

See org.eclipse.jdt.internal.compiler.ast.TypeDeclaration.createDefaultConstructor(boolean, boolean) and friends.

It should be investigated whether this could be directly generated as a `SyntheticMethodBinding` - while this is working code, it simply duplicates strategy and tempts further such creation and persistence of needlessly disparate strategies for doing the same thing. It would be good to unify the approach to synthesis of class body declarations not coded expressly in the source