Add `get_global` for `Sandbox` (#4756)

* Add `get_global` for `Sandbox`

This pr adds `get_global` to retrieve a `global` variable from an
instantiated sandbox wasm blob.

* Bump `spec_version`

* Update primitives/wasm-interface/src/lib.rs

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* `get_global` -> `get_global_val`

Co-authored-by: Sergei Pepyakin <s.pepyakin@gmail.com>
Co-authored-by: Gavin Wood <github@gavwood.com>