improve Rust bindings

[LorenzSchueler]

• make ExecutionTxContext a new type (instead of alias) which holds slices instead of pointers and lengths
• change type of input and code fields of ExecutionMessage from Option<Vec<u8>> to Option<&[u8]> thereby eliminating allocations
• no not call alloc and dealloc directly but convert between *mut T, usize and Box<[T]>
• make into_ffi_pointer safe as there is no unsafe operation involved