fix: implement ByRef with an Array Element

[boriel]

What / Why

Now it's possible to do:

DIM a(2) As Ubyte

SUB Incr(ByRef x As Ubyte)
  LET x = x + 1
END SUB

Incr(a(2))

So it's possible to pass single array elements as ByRef params whose value can be changed.

[boriel]

Fixes #994