New Intention Action: Replace with operator assign

Replaces a statement of the form a = a + b with a statement of the form
a += b.
New Intention Action: Replace with traditional assign.

This takes an expression of the form a += b and converts it to an
expression form a = a + b.
Merge pull request #386 from pk382/operator-assign-intention

Intention: Convert between "+=" and "a = a + b"
#KT-4563 Fixed
Refactoring of CompileTimeConstant, introduce flag to represent a constant being referenced by a variable