Stabilize enum `is_variant` methods as const

[CDirkx]

Recently const control flow was stabilized, which enables a lot of methods within the standard library to be made stable const; specifically all of the enum is_variant methods:

• Option: is_some and is_none
• Result: is_ok and is_err
• Poll: is_ready and is_pending
• Cow: is_borrowed and is_owned (methods themselves are unstable)
• IpAddr: is_ipv4 and is_ipv6
• std::path::Prefix: is_verbatim

This issue aims to collect discussion about the stabilization of these methods, due to their similarities and trivial implementations.

Stabilization

• Option: stabilized in PR#76135 (note: also contains as_ref)
• Result: stabilized in PR#76136 (note: also contains as_ref)
• Poll stabilized in PR#76227
• Cow: methods made const in PR#76139, the methods themselves are still unstable under cow_is_borrowed
• IpAddr: stabilized in PR#76226
• Prefix: stabilization proposed in PR#76232, but there is not enough motivation to stabilize this method on its own

[CDirkx]

These were all the instances of enums with is_variant methods I could find in the standard library.

PRs opened for all of them, @ecstatic-morse