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!
We used to treat 'rethrows' like an attribute, printing it as
'@rethrows', which was incorrect. That's fixed now, so update the
API/ABI checker tests to account for that.
