preorder, postorder, foldTree, mapTree for binary trees

[djtrack16]

preorder gets the preorder traversal of a binary search tree

postorder gets the postorder traversal of a binary search tree

foldTree is an exercise from haskellbook.com that folds/accumulates a binary tree into a value

mapTree maps a function over each value in a binary tree. Sort of like fmap for any binary tree.

This is the draft of the PR. If it is OK so far, then I will write tests for all these functions to complete the PR.

Suggestions welcome.