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