Remove SingleClient

[msvbg]

Closes: #159

The tests do not run for me, neither on main nor on this branch. I get a bunch of timeouts like these:

test matchmaking::test_lobby has been running for over 60 seconds
test networking_sockets::tests::test_create_listen_socket_ip has been running for over 60 seconds
test networking_sockets::tests::test_socket_connection has been running for over 60 seconds
test networking_types::tests::test_allocate_and_free_message has been running for over 60 seconds
test networking_utils::tests::test_get_networking_status has been running for over 60 seconds
test remote_storage::test_cloud has been running for over 60 seconds

I am also confused about this text in the Steam docs, referenced by @james7132 in the above issue:

SteamAPI_RunCallbacks is safe to call from multiple threads simultaneously, but if you choose to do this, callback code could be executed on any thread. One alternative is to call SteamAPI_RunCallbacks from the main thread only, and call SteamAPI_ReleaseCurrentThreadMemory regularly on other threads.

If I only run run_callbacks from the main thread, why would I need to release memory on other threads? And if this is a problem, isn't it also a problem with the current bindings since there is no helper for cleaning up memory on other threads?

[ceifa]

Note: This is a breaking change