Consider simplifying our PSI classes

[bjansen]

Most of the interfaces in CeylonPsi could be removed, and instead of doing:

if (is CeylonPsi.DeclarationPsi psiElement) {
    Tree.Declaration decl = psiElement.ceylonNode;
}

we could simply do

if (is CeylonCompositeElement psiElement,
    is Tree.Declaration decl = psiElement.ceylonNode) {
}

We could leave a few exceptions:

• AnyMethodPsi would satisfy PsiMethod
• AnyClassPsi would satisfy PsiClass

to be able to use our own PSI elements in APIs that expect PsiClasses and PsiMethods (see https://github.com/ceylon/ceylon-ide-intellij/issues/418#issuecomment-245893644 for example).

[gavinking]

+1.

All the value of the XxxxPsi classes is already available via the ElementType, AFAICT.