* executor: Move definitions of externals out of wasm_executor module.
* executor: Create WasmRuntime trait.
This will be used to decouple the runtime cache from wasmi execution.
* executor: Remove WasmExecutor and move methods to wasmi_execution.
These will now be crate-internal functions and there is no need
for the struct.
* executor: Set default default_heap_pages in NativeExecutor.
* cli: CLI configuration for Wasm execution method.
* executor: Remove wasmi-specific code from wasm_runtime.
* Respond to review comments.