Remove libmagic dependency

[znation]

Reasons for removing libmagic are numerous, but the main ones are:

• It's very slow
• It lies about file types
• It doesn't work on Windows

This PR replaces file type detection using libmagic with a file-extension based lookup (see file_types.rs). The lookup table is from file extension to mime type and friendly name, and uses phf for efficient compile-time construction of the hash table.

[hoytak]

I like it, +100.

• I worry that the list is a bit dated. Maybe it'd be worth writing a quick script to scrape https://en.wikipedia.org/wiki/List_of_filename_extensions_(A–E)#A, etc. to use that? We can keep it up to date that way.
• Let's do another PR after this to just replace "libmagic" with filetype or something like that.

[seanses]

I like it, +100.

• I worry that the list is a bit dated. Maybe it'd be worth writing a quick script to scrape https://en.wikipedia.org/wiki/List_of_filename_extensions_(A–E)#A, etc. to use that? We can keep it up to date that way.
• Let's do another PR after this to just replace "libmagic" with filetype or something like that.

There are extension collisions in that wiki list. Definitely need some manual screening there.

[jgodlew]

LGTM!

[znation]

@hoytak, @seanses: how about we merge this PR, then open separate issues for (1) renaming libmagic, and (2) updating the file type list using the Wikipedia data (with some manual effort)?

[hoytak]

Sounds good to me!