Runtime abstraction

[obycode]

To support Clarinet, potential future upgrades or other use cases, it seems like a good idea to abstract out the Wasm runtime instead of directly integrating wasmtime. The immediate use of this would be for clarinet to call the compiled Wasm contracts from Node.js. Doing this would require defining some trait for the execution of contracts. Each potential runtime environment needs to provide the imports for all of the host-interface functions used by Clarity contracts.

[cylewitruk]

There are some synergies here with the a/b testing tool which may be of use. We can see when it gets closer to completion what might be able to pulled out to help support Clarinet.