Add inNewFrame2

[RyanGlScott]

GaloisInc/crucible#1037 adds an inNewFrame2 combinator that is like what4's inNewFrame, but specifically for frame 2 (used for CVC5 abduction purposes). This combinator would be generally useful, and as such, we should consider upstreaming it to what4.

Note that instead of using the implementation in crucible, we should try to use the exception-safe implementation of inNewFrame as a skeleton for inNewFrame2. In fact, we can likely steal most of the implementation of inNewFrame, so we will need to figure out a reasonable way to share code between inNewFrame and inNewFrame2.