add accmulator to dfs

- add extra params to dfs/generalized and do-dfs: #:inner-init and #:combine
- non-backwards-compatible change: dfs/generalized and do-dfs
  - many input fns take an extra accum parameter
- add cc
- update tsort
- update tests
clean up do-dfs and client fns; add $acc

- non-backwards-compatible change: do-dfs #:break cannot be fn anymore
remove define-graph-property
update docs for dfs
bump to version 0.2