mv main.rs to lib.rs

[diepes]

moved main.rs to lib.rs. (and make the main() pub) replaced main.rs with a simple main() that calls drill:main().

Moving the code from main to lib.rs will be able to import the crate drill:: into tests and benchmarks. Next, I will add a benchmark, keep this pr small and focus on the code move.

Commit 1: rename main.rs to lib.rs Commit 2: add new main.rs that uses lib.rs