Tokenize parsing

[MikkelPaulson]

This is an(other) attempted solution for #104.

1. Commands are moved out of a grab bag of enums (eg. StorageCommand) into one module per command contained in a flat hierarchy, eg. crate::command::about.
2. Commands are represented by a token which can itself consist of other tokens.
3. The input string is first parsed into tokens by the parser, after which the series of tokens is passed back to the command logic to evaluate according to its own rules.
4. Metadata and tags can be associated with tokens to allow the command logic to identify which tokens were matched and respond accordingly (eg. including metadata of a matched Thing when handling the token that matched its name).

This is accomplished using async streams. The proof-of-concept mostly works now, but I'm still a bit tangled up in the ergonomics of the new setup.

See about.rs for a simple example.

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[MikkelPaulson]

@ChrisRenfrow It's actually coming together this time! Not sure how much sense you can make of this, especially in its draft form, but there will be so much less redundancy when this is done.