Deprecate spec-defined and standard-library-defined unicode operators (⇒, ←, →)

[smarter]

From the spec:

The Unicode operators \u21D2 ‘⇒’ and \u2190 ‘←’, which have the ASCII equivalents => and <-, are also reserved.

And from the standard library:

  def →[B](y: B): Tuple2[A, B] = ->(y)

While they're very cute, they're all likely to confuse users for very little benefits. And people who like symbols can simply use a coding font with special ligatures such as Fira Pro which does a much better job at solving this problem.

(It's also worth noting that → in particular is problematic because it doesn't have the same associativity as ->: scala/bug#11210)

[propensive]

I was casually wondering whether there's an appropriate Unicode character for |=>, but just removing these aliases works fine for me, too.

[ctongfei]

@propensive You have U+2907 ⤇, or a longer one U+27FE ⟾.

[He-Pin]

Currently, Akka itself seems using the shorter one by too, For me, the → is not easy to type and I had to find wtf is → in the spec. I would be great to remove these shorter things in the standard library especially when someone is the curiosity to wonder there.

[dwijnand]

For me, the → is not easy to type

(JFYI, when Akka compiles it'll reformat -> into → so you don't have to type it.)

[He-Pin]

@dwijnand I mean if I want to type it:relaxed:

使用 FastHub 从我的 Galaxy Note8 发送