Add interface to tree routines

e.g. add/remove a string, allow duplicates.
dfs, bfs, bus, leafs, visitors

Store the length of the StringSet along with the pointer.

For efficiency so we avoid having to compute its length for the lapply.

Free the data structures when they are not used.

Test this is working.

Done

Allow C routines in the lapply().

Interface for the void * user data Check.

Add ... support to the C part of the lapply.

Done.

Add the sapply to simplify the result of lapply.

Done.

Put the string names on the results of an lapply

Done.