Make all bounded channels safe

[mahkoh]

This can be done by checking on 32 bit systems if overflow occured after we've acquired a read/write slot. If it happened we have to abort the process. On 64 bit overflow can be considered impossible.

[marcusklaas]

This is probably extremely naive, but can't we work with 64 bit values regardless of architecture? Or do (some) atomic instructions only work with native word sizes?

[mahkoh]

Unfortunately the stdlib only provides usize atomics.