Contract signatures checking (#478)

* Convert T in vm::Ext to a associated type

* WIP

* Fix BTreeMap

* Extract prepare module from vm.

* Move tests as well.

* Fix doc comment.

* macro for env defintion

* Fix prepare tests.

* Clean up

* Renamings

* Refactor scan_imports

* Improve docs

* Docs.

* Add some tests for sandbox module

* Clean up

* Use Error::Instantiate instead of ::Deserialize

* Add test for imports

* Add wrong sig import

* Clean up

* Rebuild binaries.

* Use "another_module" instead of obscure "vne"

Since "vne" looks like an actual typo

substrate/Cargo.lock

@@ -2795,6 +2795,7 @@ dependencies = [
 name = "substrate-runtime-sandbox"
 version = "0.1.0"
 dependencies = [
+ "assert_matches 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_version 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "substrate-codec 0.1.0",
  "substrate-primitives 0.1.0",