show scheme parameters hash

Provide some way to show a scheme's parameters hash. Can be useful when:

I have an Arc scheme, but not the one currently-deployed. I want to assign it to the DAO using the same scheme parameters as the scheme it is replacing.

(Perhaps in this scenario a "replace scheme" tab in SchemeRegistrar would also be handy)

In any case, it can be useful in general to provide this information to script developers.

No files, tests, or entry points are named. Start by tracing how Arc schemes and SchemeRegistrar are represented, then determine where the parameters hash should be exposed for script developers. Done means the hash can be obtained for an existing scheme and used when assigning a replacement.

Written by the indexing model from the issue text.