remove spawn from top-level

[yoshuawuyts]

This removes the spawn function from the top-level exports

Instead task::spawn should be used. If this were part of std, spawn
wouldn't be a top-level method, so we should keep it in task instead.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[ghost]

Looks good to me! One thing I would note is that runtime::task::spawn is little verbose. Perhaps that'd look better if runtime::task was imported first and then we spawned tasks with task::spawn?