Add Constant C-style String Propagation

[burz]

The following changes allow users to propagate constant C-style string values over the control flow graphs of their code.

@ruoso

[ruoso]

Do we need to make all those interfaces public? Isn't there a part that can be made implementation-private?

[burz]

@lukaszkolodziejczyk I spoke with @ruoso about this and he said that he preferred it this way, not that I don't agree.