Iterator interface for picks, and minor code cleanup.

[jswrenn]

This pull requests implements IntoIterator for &AliasTable, and introduces AliasTableIterator (which implements Iterator).

A few minor code-smells are corrected, too:

• replaced two multi-line routines with more succinct calls to standard-library functions (namely extend)
• replaced the num dependency with num_traits, the only subset of num that was used
• made clippy happy

I use the pick iterator here.

[sdleffler]

Nice work! Thanks for the PR.