autoload -- arrange for a function to be loaded automatically

i1 : fn = temporaryFileName()

o1 = /tmp/M2-7857-0/0

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

o2 = /tmp/M2-7857-0/0

o2 : File

i3 : autoload(f,fn)

i4 : code f

o4 = /usr/share/Macaulay2/Core/autoload.m2:5:20-11:23: --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.m2:5:13-5:14
     | sym      : Symbol          -- f                  /usr/share/Macaulay2/Core/autoload.m2:3:29-3:32
     | filename : String          -- "/tmp/M2-7857-0/0" /usr/share/Macaulay2/Core/autoload.m2:3:33-3:41

i5 : f 4

o5 = 5

i6 : removeFile fn