Minor changes

Added find and getAll (in ascending order) functions to OptimalBST.
Added ArrayList-forming utility functions to ComparableAdaptor and
StaticNode and implemented them in HuffmanTree (removing the need for
the Symbol class) and RandomEncodingTree.
Added/fixed valid input checks in EncodingTree, HuffmanTree, OptimalBST,
RandomEncodingTree, StaticTree, and Utilities.
Condensed some code in HuffmanTree and RandomEncodingTree.