Adds isInstance flag

[markwpearce]

Adds the isInstance flag on symbol extra data for when a symbol represents an instance of a type.

This is useful for walking all variable expressions, and being able to check if that expression is an instance of the type or a reference to the actual type (or class constructor).

eg:

class Klass
end class

sub test(k as Klass)  ' k is an instance, Klass is not, but both are of ClassType("klass")
end sub

[markwpearce]

Example of use of the isInstance flag