Incompatible change: no longer treat all function calls as method calls

Previously it depended on the first parameter if a P5 function would be called
as function or as method. We now treat functions and methods differently.
$p5.call is now used only for functions.
$p5.invoke is now used for all methods, i.e. package methods and object methods.

This commit also reverses the first two parameters to invoke so they are now
package/object, name which should be much more natural.