Generalized valid arguments for a multi arg message a bit. Now, access
(declared variables and arguments, and self slots) and par-args are also
valid.

Prior to this, the following was necessary:

var str = "Hi there"
write(str) // Notice the parenthesises

Now, the following corresponds to the above:

var str = "Hi there"
write str // Notice the absence of parenthesises

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>