Add `send_non_blocking`

Right now DSLX proc's will always stall on `send`, if no other proc is actively waiting on `recv`.
This approach makes so no data will be lost/dropped, but it takes away control from users.

I propose adding `send_non_blocking` and `send_if_non_blocking`. This way send
will not block pipeline and users will be able to decide, what they want to do with data, like:
* dropping it,
* buffering it,
* sending it over other channel,
* or something else.

These functions would have following signatures:
* `send_non_blocking(:token, :chan<:T>, :T) -> (:token, :u1)`,
* `send_if_non_blocking(:token, :chan<:T>, :u1, :T) -> (:token, :u1)`.

Second element in the tuple is 0/1 flag. If transfer was successful flag
would be high, otherwise it would be low.

/cc @proppy