Introduce a marker protocol SendableMetatype that is used to indicate
when the metatype of a type will conform to Sendable. Specifically,
`T: SendableMetatype` implies `T.Type: Sendable`. When strict
metatype sendability is enabled, metatypes are only sendable when `T:
SendableMetatype`.
All nominal types implicitly conform to `SendableMetatype`, as do the
various builtin types, function types, etc. The `Sendable` marker
protocol now inherits from `SendableMetatype`, so that `T: Sendable`
implies `T.Type: Sendable`.
Thank you Slava for the excellent idea!
The stdlib is always built with NoncopyableGenerics enabled, so `#if
$NoncopyableGenerics` guards in non-inlinable code are superfluous.
Additionally, the stdlib's interface no longer needs to support compilers
without the feature, so the guards in inlinable code can also be removed.
