This commit adds `cargo update` to the distclean target in the
makefile. This updates the Cargo.lock file when clearing the
deps directory.
In addition, it adds a faster implementation of the Sieve of
Eratosthenes for use by `src/factor/gen_table.rs` and `test/factor.rs`.
Add a test for `factor`.
This commit also pulls factor's Sieve implementation into its own module
so that the factor test can use it.
Finally, slight refactoring for clarity in gen_table.rs.
