Deprecate libnum in favor of rust-lang/num

This is part of the migration of crates into the Cargo ecosystem. There
is now an external repository https://github.com/rust-lang/num for bignums.

The single use of libnum elsewhere in the repository is for a shootout
benchmark. This commit does not attempt to remove that usage for now.

Due to deprecation, this is a:

[breaking-change]