Future direction: Non-copyable `Span`

Given that `Span` should be ended exactly once, and should not be shared between subprocesses or preserved after termination, it seems a potential client of the recently accepted [SE-0390](https://github.com/apple/swift-evolution/blob/main/proposals/0390-noncopyable-structs-and-enums.md). With Swift 5.9+ we can mark `Span` as non-copyable, and only allow the operation to “borrow” the span. `span.end()` will consume the span. We can also add `span.end()` in `deinit` to automatically terminate the span.

Note that this is unfortunately **very breaking**. Haven’t thought about if there’s a way to land this safely in the 1.x period.

Start by reading the issue and the linked SE-0390 proposal to understand the proposed non-copyable Span and consuming end() semantics. Determine whether this can be adopted safely during the 1.x period despite the stated breaking changes; done means a decided, implementable direction rather than an unresolved future-design discussion.

Written by the indexing model from the issue text.