Implement the `svm_transfer` host function

[YaronWittenstein]

Add a new host function that will send coins from the current executing account to a destination account.

The host function should live under: https://github.com/spacemeshos/svm/tree/master/crates/runtime

Notes

• The code should integrate with the AccountStorage (wraps the Global State): https://github.com/spacemeshos/svm/blob/master/crates/state/src/account_storage.rs
• If the destination account doesn't exist, then the host function should panic. This behavior might change in case the Self Spawn feature is implemented.

Testing

Testing the Host Function in isolation

Under the svm-runtime there're tests that exercise the host functions without creating a full-blown SVM Runtime.

Here is the file containing such tests: https://github.com/spacemeshos/svm/blob/master/crates/runtime/tests/vmcalls_tests.rs

Now, in order to generate Wasm inputs for the tests, please look at the current ones: https://github.com/spacemeshos/svm/tree/master/crates/runtime/tests/wasm

Other Tests

• Testing a Template running within the SVM Runtime

This is not a requirement for that issue. But it's something we'll want to have. One way to achieve that will be by extending this test:

https://github.com/spacemeshos/svm/blob/478d12fddfa87e6230a64010bfd3c0f24ad3164f/crates/runtime/tests/runtime_tests.rs#L311

• Testing the SDK - See issue: #407

[neysofu]

Closed by #419.