Make BranchHandler.VerifyCondition public and pure

[Washi1337]

Summary:
In a lot of cases, it can be useful to determine whether a branch is taken or not, without actually emulating the branch instruction itself.

Currently, derivatives of BranchHandler use VerifyCondition to pop the values from the stack and perform the check. The proposal is to make this method public, and peek values from the stack instead of popping them, such that it can be invoked multiple times without affecting the current program state.

Additional context:
Requires #97