Add Declaration

No subtypes yet… there’s kind of a circular dependency between
Declaration and Body. I’ll properly add Body next, and then some proper
declaration types.

CC #46.
Add tests for Declaration backend conversion

It’s a joke really, but more gravely, a test that does not pass. I can’t
properly implement it without a subtype to test :-( hopefully this
shouldn’t happen too often.
RedHat backend: clarify, add utility function

Clarify that statementToCeylon cannot be used for declarations (even
though declarations are statements in the RedHat AST), and add a
function that *can* be used for both statements and declarations.
Add Body, Block, ClassBody, InterfaceBody

Pretty straightforward, except that the abstract Body can’t be compiled
– utterly ambiguous!
Add tests for Body, Block, ClassBody, InterfaceBody