Write successors to arbitrary depth in Racket.
Inspired by this tweet,
which simply wrote "succccccccc" in response to
a posting of the cadnr Racket package.
Installs a #%top macro that
checks whether an identifier fits the syntax of a successor and,
if so, converts it to one. If the same identifier name is bound in
the module, however, the behavior depends on the order of binding.
succ.rkt: core functions
succ-test.rkt: Test suite for the above
succ-macros.rkt: core macro
succ-client.rkt: examples of use