Adds new execution strategy nativeElseWasm (#1546)

* fix: adds new execution strategy nativeElseWasm and replace nativeWhenPossible with it

* feat: adds cmd line params for execution strategies

* fix: uses of cmd line execution strategies

* chore: remove white spaces

* chore: remove println

* chore: remove whitespace

* fix: generating functions with context

* feat: add function to generate with_context declarations

* fix: add implementation for with_context function calls

* fix: add execution context to call_api_at function

* fix: making use of context to select strategy for block_builder

* chore: cleaning up

* fix: merging issues

* fix tests

* add wasm files

* chore: small doc for context fields

* chore: delete redundant docs

* fix: use full path for ExecutionContext

* fix: add context functions from inside fold_item_impl

* chore: remove clone

* fix: moving generative function to utils, remove unused imports

* fix: add missing full path for ExecutionContext

* fix: merge issues

* update wasm files

* fix: update to keep up with changes in master

* chore: remove unused functions, clean up

* fix test

* fix grumbles

* fix: add more tests

* fix: some refactorings

* feat: add execution strategy to call

* chore: small improvements

* fix: add message to panic

* fix tests

substrate/core/service/test/src/lib.rs

@@ -28,7 +28,6 @@ use tokio::timer::Interval;
 use primitives::blake2_256;
 use service::{
 	ServiceFactory,
-	ExecutionStrategy,
 	Configuration,
 	FactoryFullConfiguration,
 	FactoryChainSpec,
@@ -118,8 +117,7 @@ fn node_config<F: ServiceFactory> (
 		chain_spec: (*spec).clone(),
 		custom: Default::default(),
 		name: format!("Node {}", index),
-		block_execution_strategy: ExecutionStrategy::NativeWhenPossible,
-		api_execution_strategy: ExecutionStrategy::NativeWhenPossible,
+		execution_strategies: Default::default(),
 		rpc_http: None,
 		rpc_ws: None,
 		telemetry_url: None,