Adopting `libdebug` removal

The only thing which is confusing me is forcing 
`fmt::Show` trait for `bwt`-related things. But in
fact it should never be the issue as `NumCast` is 
so far implemented for primitive types and they
definitely have also `fmt::Show` implementation.
Merge pull request #34 from vhbit/debug-removed

Adopting `libdebug` removal