This commit updates all helper functions, taking into account recent
developments regarding `tokio`.
In particular, the `block_in_place()` and `block_on()` functions now
don't panic anymore if used within the single-threaded `tokio` runtime
and instead behave as expected in both runtime flavours.
Furthermore, because `tokio` may add more runtime flavours in the
future, all helpers will now panic if used within an unsupported
runtime. This is to prevent unforeseen behavioural quirks and
interactions with `tokio` internals.
The above changes make `BlockingGuard` redundant; it is consequently
removed.
The documentation is also updated, describing the behaviour of the
helper functions and the purpose of the `runtime.rs` module in more
detail.
Signed-off-by: Max Carrara <m.carrara@proxmox.com>
