At the moment `Ap` state contains vector of generations, although the ap instruction itself allow to specify only one stream. It was done to support several streams in a result position in a future. But the current realization allows to handle empty vector that was implemented to support states for scalars and it turned out (for more info see #326) that states for scalars aren't actually needed.
Closes#355.
This PR refactors call merger of `TraceHandler`. Previously it requires `CallOutputValue` to determine a type of call output value (stream or scalar). And internally it checked correspondence between data result and call output type and return a error if they are not equal. Although execution engine component also had a match over these values and does nothing if they are not matched since `TraceHandler` did this job. This PR eliminate such behaviour and improve isolation of AquaVM modules.
feat(execution-engine): change behaviour of fold over streams
Change behaviour of fold over streams to make it more similar to pi-calculus channels/names (for more info see #333).
Closes#333.
BREAKING CHANGE:
The new stream behaviour is not compatible with old one, such as
```
(fold $stream iterator
(seq
(call ...)
(next iterator)))
```
will never end after this change (for more info again see #333).
* Get rid of unsafe code unless really necessary
* Add lint levels where appropriate
Some crates (air-beautifier, air-testing-framework) have lot of
rust_2018_idioms violations, that will be resolved later.
Many internal structures refer to trace positions; this is an important type of values. In the code, it is sometimes `u32`, sometimes `usize`. While such variables usually have "_pos" suffix, having a dedicated type does improve code with type guarantees.
Intially, a fold instruction was designed based on idea that its iterable can't expand,
so AIR was considered as a subtree. But after introducing recursive streams in #225, it's
no more a subtree, but a subgraph.
