Draft for a Listener Interface on IRScopes:

1. add of instr to the scope
2. begin and end (with its Operand result) of IRBuilder.build()

1. enables traceability from scope to its instructions and 2. the mapping of an
IRScope to its (AST)Node
Wrapper for notfications on a List<Instr> (used in BasicBlock)
Listener on changes of a List<Instr> for instance inside of a basic block:

1. models the operation of modification
2. callback function to specify what going to change