Move script flag to/from-string logic from tests to script/interpreter #10730

When we last discussed making scripts debuggable (sometime after #3901), the plan was to instead trace execution rather than single-step through it.

This is a dependency step toward that, and is eventually used for the RPC verifyscript to parse its "flags" option.

See also #10729

Needs rebase

Rebased

I'm okay with moving this out of the tests if it's necessary in another place, however the from/to string stuff doesn't belong with the interpreter consensus code IMO. Better to keep it in a separate compilation unit, like with the CBitcoinAddress split done in #11117.

@luke-jr ping?

Hmm, so where would you suggest? Bundle it in with RPC-specific stuff?

Rebased, and moved it to script/script_flags.{h,cpp} for now.

Closing due to apparent lack of interest/reviewers