Make -> sugar an inline method

[bishabosha]

Must wait for 3.10.0

currently it is transparent inline, to avoid inline proxy objects appearing in capture sets and breaking type checking.
now it follows the design:

extension [A](inline self: A) inline def ->[B](inline that: B): (A, B) = (self, that)

based on #25175

requires to introduce the change in src-bootstrapped (until rebootstrapping)

fixes #24715

[bishabosha]

I made it transparent inline because this error shows up when only using inline: (see #24729)

[error] -- [E007] Type Mismatch Error: .../library/src/scala/collection/package.scala:71:16
[error]  71 |      else Some(t.head -> t.tail)
[error]     |                ^^^^^^^^^^^^^^^^
[error]     |      Found:    (A^'s1, C^{y$proxy1})
[error]     |      Required: (A, C)
[error]     |
[error]     |      Note that capability y$proxy1 is not included in capture set {}.
[error]     |---------------------------------------------------------------------------
[error]     |Inline stack trace
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |This location contains code that was inlined from Predef.scala:616
[error] 616 |    inline def -> [B](y: B): (A, B) = (self, y)
[error]     |                                      ^^^^^^^^^
[error]      ---------------------------------------------------------------------------
[error]     |
[error]     | longer explanation available when compiling with `-explain`

[bishabosha]

#24728 (comment) This should really be a non-transparent. If it bothers CC, then CC must adapt.

ok then lets say blocked by #24729

now thats fixed

[hamzaremmal]

I believe it's better to put it in the package object scala instead.

[bishabosha]

Why?

[bishabosha]

scala package is really only for exporting forwarders is it not?
-> is an additional sugar

[Gedochao]

@bishabosha might be worth a rebase, we're entering 3.10.

[bishabosha]

@Gedochao rebased

[bishabosha]

potential problem, it looks like in 830590e that trying to import from Predef explicitly will bias in favor of ArrowAssoc and cause deprecation warning, and then it isnt possible to unimport ArrowAssoc => _ without typer no longer resolving -> extension method