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/rpc/Cargo.toml

@@ -19,6 +19,7 @@ client = { package = "substrate-client", path = "../client" }
 substrate-executor = { path = "../executor" }
 network = { package = "substrate-network", path = "../network" }
 primitives = { package = "substrate-primitives", path = "../primitives" }
+state_machine = { package = "substrate-state-machine", path = "../state-machine" }
 transaction_pool = { package = "substrate-transaction-pool", path = "../transaction-pool" }
 runtime_primitives = { package = "sr-primitives", path = "../sr-primitives" }
 runtime_version = { package = "sr-version", path = "../sr-version" }