Export loadContract method of runtime.interpreterEnvironment type

[m-Peter]

Issue to be solved

As discussed in this onflow/cadence-tools#210 (comment), it would be useful to directly call this method, having obtained a runtime.interpreterEnvironment object, e.g. with:

env := runtime.NewBaseInterpreterEnvironment(config)

Right now, we have to duplicate the logic of this method, when using a custom handler for:

env.InterpreterConfig.ContractValueHandler

Suggested Solution

Export the loadContract method, possibly via an interface, since runtime.interpreterEnvironment is not an exported type, and cannot be used for example as a struct field.