Last use analysis for array short circuiting

Last-Use analysis of a Futhark program in aliased explicit-memory lore form. Takes as input such a program or a function and produces a 'M.Map VName Names', in which the key identified the let stmt, and the list argument identifies the variables that were lastly used in that stmt. Note that the results of a body do not have a last use, and neither do a function parameters if it happens to not be used inside function's body. Such cases are supposed to be treated separately.