Only native host functions

[YaronWittenstein]

The executed SVM-flavored Wasm has imports of function kind.
New kinds of imports (such as Memory/Table/Global) are not expected to be used in the foreseeable future.

The functions imports are grouped into:

SVM Internals:

These host functions deal primarily with managing the Account storage.
The internals are SVM specific and are Node-agnostic.

The internals will be clustered under the svm namespace.

SVM Externals

Here we can inject upon Runtime initialization a functionality that is Node-aware.
It will include features such as:

• get_balance - asking for the current executed Account balance
• transfer - sending coins from the current Account to another Account

The externals will be clustered under the sm namespace.

This design should make it easier for other Blockchains to integrate with SVM.
The only requirement is that integrating Blockchain Node will provide functions written in Rust.
(and have a type signature abiding to the Wasmer native host function with env).

Notes:

• This PR didn't tackle the #[ignore] tests under the runtime-ffi crate.
• The externals of SVM will be added incrementally. Most of them will talk directly to the upcoming Global-State written in Rust.
• While working on the code I've started using Account replacing App (see Replacing the term App with Account #264) and did some refactoring along the way.

Depends on:

• FFI Imports Removal #269