Adaptive heap size (#328)

* heap-size is a CLI arg, make it 512 by default

* Fix tests

* Adaptive heap size.

* Allow storage_exists

substrate/polkadot/cli/src/cli.yml

@@ -99,6 +99,14 @@ args:
       long: execution
       value_name: STRATEGY
       help: The means of execution used when calling into the runtime. Can be either wasm, native or both.
+  - min-heap-pages:
+      long: min-heap-pages
+      value_name: COUNT
+      help: The number of 64KB pages to allocate for Wasm execution initially.
+  - max-heap-pages:
+      long: max-heap-pages
+      value_name: COUNT
+      help: The maximum number of 64KB pages to ever allocate for Wasm execution. Don't alter this unless you know what you're doing.
 subcommands:
   - build-spec:
       about: Build a spec.json file, outputing to stdout
@@ -162,3 +170,15 @@ subcommands:
               value_name: PATH
               help: Specify custom base path.
               takes_value: true
+          - execution:
+              long: execution
+              value_name: STRATEGY
+              help: The means of execution used when calling into the runtime. Can be either wasm, native or both.
+          - min-heap-pages:
+              long: min-heap-pages
+              value_name: COUNT
+              help: The number of 64KB pages to allocate for Wasm execution initially.
+          - max-heap-pages:
+              long: max-heap-pages
+              value_name: COUNT
+              help: The maximum number of 64KB pages to ever allocate for Wasm execution. Don't alter this unless you know what you're doing.