Implemented LEAST function with variable number of arguments.

Implemented LEAST function with variable number of arguments and deleted its old implementations. It works in presto-client, but old unit tests for Least function can't make a call to it now. 
undefined behavior. this will work for a lot of types (anything where the stack encoding happens to match Java's): See this guide I wrote. Particularly the section on types: https://github.com/facebook/presto/pull/1964