Generalize `spawn` beyond unit closures

`thread::spawn` was previously restricted to closures that return `()`,
which limited the utility of joining on a spawned thread. However, there
is no reason for this restriction, and this commit allows arbitrary
return types.

Since it introduces a type parameter to `JoinHandle`, it's technically
a:

[breaking-change]

However, no code is actually expected to break.

0 files changed, 0 insertions, 0 deletions