autoload -- arrange for a function to be loaded automatically

i1 : fn = temporaryFileName()

o1 = /tmp/M2-499651-0/0

i2 : fn << "f = x -> x+1\n" << close

o2 = /tmp/M2-499651-0/0

o2 : File

i3 : autoload(f,fn)

i4 : code f

o4 = /usr/share/Macaulay2/Core/autoload.m2:8:20-14:23: --source code:
          sym <- f := x -> (
               load filename;
               if f === value sym 
               then error("symbol '", toString sym, "' didn't acquire a new value in file '", filename, "'");
               (value sym) x
               );
          )
     | symbol     class              value                location of symbol
     | ------     -----              -----                ------------------                                        
     | f        : FunctionClosure -- ...                  /usr/share/Macaulay2/Core/autoload.m2:8:13-8:14
     | sym      : Symbol          -- f                    /usr/share/Macaulay2/Core/autoload.m2:6:29-6:32
     | filename : String          -- "/tmp/M2-499651-0/0" /usr/share/Macaulay2/Core/autoload.m2:6:33-6:41

i5 : f 4

o5 = 5

i6 : removeFile fn