svm-runtime-ffi tests restoration

[YaronWittenstein]

This #269 PR made the svm_runtime_success made svm_runtime_failure obsolete.

This PR re-implements both tests under the new design.

Notes:

• Since there is no option to inject imports using the FFI layer anymore, the Rust examples using the SVM SDK have been updated as well.

• Both svm_envelope_alloc and svm_context_alloc have been modified not to accept any input.
  The binary size of Envelope and Context is fixed.

• When encoding an Error Message as part of a Receipt, the message is truncated to be of length 255 bytes at most.
  (the classic example, is having an error message containing a deep call-stack).

[neysofu]

Some changes I'd like to see:

• Remove unsafe from svm_envelope_alloc, svm_message_alloc, svm_context_alloc. They're not needed (not introduced with this PR but since we're at it we might as well do it).
• Remove svm_byte_array::bytes. It's equivalent to x.as_slice().as_ptr(), which is a safe alternative.
• Replace svm_byte_array::bytes_mut with a safe fn as_slice_mut(&mut self) -> &mut [u8] { ... }. Note the added safety and how it requires &mut.
• Safer byte_array_copy:

fn byte_array_copy(byte_array: &mut svm_byte_array, bytes: &[u8]) {
    byte_array.as_slice_mut().copy_from_slice(bytes);
}

This reflects in tests under api_tests.rs e.g.:

let mut byte_array = api::svm_message_alloc(msg.len() as u32);
byte_array_copy(&mut byte_array, &msg);