Add a new `Alias` struct and implementation

This allows us to represent an alias not just as a string, but as an
entity where we can ask for and isolate different parts of the alias.
This makes it more convenient to get just the "alias" part, or just the
"email" part for example. Doing so is necessary for making accurate
comparisons/searches to find out whether this email or alias already
exists in our alias file.

Two functions are implemented on this type. One, `to_string`, creates a
Mutt alias line as a string. The other builds a new `Alias` object from
a "From: " string, taking nearly the same steps as `build_alias` does.

We'll want to transition `build_alias` to this `Alias::new` function,
using the new one in the future.

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