implement recursive alias resolution

[reubeno]

Today, brush has a quick and dirty implementation of alias resolution in interp.rs that works well enough for very simple cases (e.g., alias ll='ls -l'. Per the POSIX spec, alias resolution is supposed to happen after tokenization but before token recognition ("parsing"). It also needs to handle resolving an alias defined in terms of another alias, as well as some cases where subsequent tokens in the command line need to be processed as well.

There's a test case in alias.yaml that currently shows an easy-to-reproduce failure because of missing recursive logic.