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, which is being moved into the external crate.

Due to deprecation, this is a:

[breaking-change]

src/etc/licenseck.py

@@ -49,7 +49,6 @@ exceptions = [
     "test/bench/shootout-mandelbrot.rs", # BSD
     "test/bench/shootout-meteor.rs", # BSD
     "test/bench/shootout-nbody.rs", # BSD
-    "test/bench/shootout-pidigits.rs", # BSD
     "test/bench/shootout-regex-dna.rs", # BSD
     "test/bench/shootout-reverse-complement.rs", # BSD
     "test/bench/shootout-threadring.rs", # BSD