Disallow polymorphic function types over impure function types

[odersky]

Disallowed and rejected with an implementation restriction error:

[A] => B => C

OK:

[A] => B -> C

Also OK (and equivalent to last one):

[A] -> B -> C

This is necessary since our current polymorphic function encoding requires that the type that's quantified over is a pure function type. We should fix that but until that's done we need to enforce the restriction.

Fixes #26035

[odersky]

@bracevac There are some scaladoc tests failing, which probably means scaladoc pages are now wrong as well. Can you take a look?

[bracevac]

@bracevac There are some scaladoc tests failing, which probably means scaladoc pages are now wrong as well. Can you take a look?

It's because the rendering test captureCheckingSignatures.scala tests some lambdas that are now illegal. These just need to be commented out:

Error:  -- Error: /home/runner/work/scala3/scala3/scaladoc-testcases/src/tests/captureCheckingSignatures.scala:82:22 
Error:  82 |  val polyPureV2: [A] => Int => A ->{a,b,c} Int //expected: val polyPureV2: [A] => Int => A ->{a, b, c} Int
Error:     |                      ^
Error:     |Implementation restriction: polymorphic function types cannot wrap impure function types if capture checking is enabled
Error:  -- Error: /home/runner/work/scala3/scala3/scaladoc-testcases/src/tests/captureCheckingSignatures.scala:83:23 
Error:  83 |  val polyImpureV: [A] -> A => Int //expected: val polyImpureV: [A] => A => Int
Error:     |                       ^
Error:     |Implementation restriction: polymorphic function types cannot wrap impure function types if capture checking is enabled
Error:  -- Error: /home/runner/work/scala3/scala3/scaladoc-testcases/src/tests/captureCheckingSignatures.scala:84:24 
Error:  84 |  val polyImpureV2: [A] -> A => Int //expected: val polyImpureV2: [A] => A => Int
Error:     |                        ^
Error:     |Implementation restriction: polymorphic function types cannot wrap impure function types if capture checking is enabled

[odersky]

@bracevac Good suggestions. Can you take the PR over?