Sandboxing and the simplest smart-contract runtime (#140)

* Add primitives for sandboxing.

* Add sandbox module.

* Implement the runtime part of the sandbox.

* Rebuild binaries.

* Implement smart-contract execution.

* Add more documentation.

substrate/demo/runtime/wasm/init.sh

@@ -4,3 +4,4 @@ rustup update nightly
 rustup target add wasm32-unknown-unknown --toolchain nightly
 rustup update stable
 cargo install --git https://github.com/alexcrichton/wasm-gc
+cargo install --git https://github.com/pepyakin/wasm-export-table.git