Added implemention of build in ASTBuilderFromReceiver, where receiver is
supplied as ASTThis.INSTANCE.
Corrected wrapIntoClosure to wrap arg into ExpressionReceiverContext
before adding to ExpressionContext.
Corrected implementation of visitAssignment to build ASTBehavior's.

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