Simplify ResourceFile._references and ._LazyResourceMap
Simplify ResourceFile._references and ._LazyResourceMap

The _references map now stores Resource objects directly, instead of
constructing them only when they are looked up. Resource objects are
now lazy themselves, so the previous lazy resource creation mechanism
is redundant.

_LazyResourceMap is now a simple read-only wrapper around an existing
map. The custom class is now only used to provide a specialized repr.

dgelessus/python-rsrcfork