Added tests for generation of the exists expression
Added the exists expression to the IR AST and grouped it under the same interface type as the forall expression
Introduced generic functionality for generation of the forall and the exists expressions into nodes of the IR AST
Updated constants with name prefixes used for generation of the exists expression
Added generation of the exists expression into a node of the IR AST. Also, made the generation of the forall expression use the newly introduced generic functionality in the expression assistant.
Changed the strategy used for transformation of the forall expression into a generic strategy that will handle both the forall expression and the exists expression