Implement basic resource equivalence

[otaviomacedo]

To detect which resources should be refactored, we need to indentify which
resources have only changed their location, but have remained "the same", in
some sense. This can be made precise by defining an equivalence relation on
the set of resources.

Before that, let's define a digest function, d:

d(resource) = hash(type + properties + dependencies.map(d))

where hash is a cryptographic hash function. In other words, the hash
is computed from its type, its own properties (that is, excluding properties
that refer to other resources), and the digests of each of its dependencies.

The digest of a resource, defined recursively this way, remains stable even if
one or more of its dependencies gets renamed. Since the resources in a
CloudFormation template form an acyclic graph, this function is well-defined.

The equivalence relation then follows directly: two resources r1 and r2
are equivalent if d(r1) = d(r2).