Change sender struct selectors to receiver struct selectors

[emil14]

Right now we have syntax

$msg/foo/bar -> ...

But what if we need /bar in one place and /baz in another? We have to do this

$msg/foo/bar -> ...
$msg/foo/baz -> ...

That's not a big deal itself but isn't consistent with "multi-receivers" syntax sugar like this which we have for senders without struct selectors

$msg -> {
  a.b
  c.d
}

We can do this

$msg/foo/bar -> {
  a.b
  c.d
}

But we can't do this

$msg -> {
  /foo/a.b
  /bar/c.d
}

Actually we can implement this but there's two reasons why not to

1. This syntax isn't obvious
2. It's not consistent with senders version where we have <SENDER>/<SELECTORS> like a.b/c/d where a.b is sender and /c/d are selectors

This leads as to these options

1. Have not-consistent syntax
2. Have only senders sugar
3. Have only receivers sugar
4. Re-implement senders sugar so it will be consistent with receivers syntax

Let's start with the number 4 option which would be perfect. The problem is - with senders we want selectors right after port address. Because selection is something that happens after message is sent. With receivers we want selectors before port address because selection is something that happens before final message (struct field) is received.

This leads us to conclusion that we only have 3 options:

1. Have not-consistent syntax
2. Have only senders sugar
3. Have only receivers sugar

---

Having only receivers (option №3) have benefits over having only senders (option №1) because we can say this

a.c -> {
  /foo/ d.e
  /baz/ f.g
}

And be consistent because we don't have senders to be inconsistent with.

The problem case is when we send same field to different places:

a.c -> {
  /foo/ d.e
  /foo/ f.g
}

---

Option number 1 is possible too if only it won't make grammar complicated

a.c/foo/bar -> {
  /baz d.e
  /bax f.g
}