Expose SDK funtionality to allow easier creation of custom runtimes

[edwardpeters]

Feature Request

Description of Problem:

Ability for users to create custom runtimes which re-use existing SDK methods, evaluation loop and Native function utilities. (Currently these options exist but are marked private[runtime]).

One use case of this is to allow some semblance of FFIs, tho only to the user extending the runtime (not anyone calling the extension.)

Potential Solutions:

Minimal PoC exists in https://github.com/edwardpeters/morphir-scala-interpreter/tree/ffi-poc

Simply removing some private markers would be sufficient to let users write their own extension functions.

• This would allow the extended runtimes to have safety issues such as nondeterminism (tho would not pass that risk on to the users of those runtimes)
• This would expose some limited implementation details (tho many of the details are already exposed, as the structure of RTValue has wound up in "public" code)