Add IdentityTransform

This adds an identity transform that applies an identity function to some
CircuitState, i.e., it just returns the original CircuitState. This is
useful for transform generators that may, for edge cases, generate an
empty transform sequence. Other classes (e.g., Compiler) have explicit or
implicit requirements that a transform sequence is non-empty.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>