state package

[emil14]

Implement state package with 3 public components:

1. state.State - main component, must be implemented as runtime-function
2. state.Bool - little helper that wraps State and implements boolean routing
3. state.Accumulator - this one needs to be moved from builtin/streams.neva (and should be imported from there), it's used by Reduce (in the future it will be moved to streams.Reduce)

// State maintains the value of type T.
// It waits for message from `init` and `get`, then sends initial value to `res`.
// After that loop starts - wait for `set` and `get`, send latest value to `res`, repeat.
// It only receives `init` once, subsequent messages to that port will be blocked.
#extern(state)
pub def State<T>(init T, set T, get T) (res T)

// Bool is a helper-component for routing based on a boolean state.
// It wraps `State<bool>` and has the same semantics except conditional output.
pub def Bool(init bool, set bool, get bool) (then bool, else bool) {
    state State<bool>
    ---
    :init -> state:init
    :set -> state:set
    :get -> state:get
    state:res -> switch {
        true -> :then
        _ -> :else
    }
}

// Accumulator maintains the current state of the reduction.
// It updates its value with each new input and outputs the final result when last is true.
#extern(accumulator)
pub def Accumulator<T>(init T, upd T, last bool) (cur T, res T)