This PR moves FindAndDelete and GetOp2 out of CScript (the first is only used inside the interpreter and moved there, the second does not actually depend on any script specifics and works on any vector). Furthermore, all non-const-iterator versions of GetOp are replaced by const ones, removing a number of methods in the process.
The longer term goal here is making the script interpreter independent from the CScript representation.
Note for reviewers: both FindAndDelete and GetScriptOp are consensus critical.