Add ternary search trie implementation

Simple trie requiers R(alphabet size) links including NULL link on every
node. It leads to out of memory issue on certain cases. TST node contains
only three links - left(lesser), middle(equal) and right(greater). Here is
tries summary. So it requires only O(4N) space, while simple trie
implementation is O((R+1)N). Time complexity for search hit is O(L+lnN),
search miss - O(lnN). Possible improvement is R^2 branching on root
node: R^2 way branching done at root and each of R^2 root nodes points to
a TST. TST with R^2 space complexity is O(4N+R^2).