[Iguana] Move ConditionStack to conditionstack.h
Summary:
To build ScriptInterpreter, we need to have access to ConditionStack in interpreter.h.
The simplest way is moving this into its own header file. The class is so simple that it doesn't need a corresponding cpp file.
Test Plan: ninja check
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D16766