ENH: add dictionary searcher methods

- csearch(), search(), csearchScoped(), searchScoped() methods
  can be used to find an entry and also retain the enclosing
  dictionary context.

- To avoid ambiguity between a dot (.) as a scoping character and
  legal part of a keyword, entries can now be accessed directly using
  slash-scoping semantics similar to file-names:

    * cfindScopedDictPtr, findScopedDictPtr

- To get or create a sub-dictionary entry:

    * makeScopedDictPtr, which behaves similarly to mkdirp in that it
      will create any intermediate dictionaries as required.