replace git-sha1 with simple-sha1

[astro]

See: #234

I would like to use the native crypto module in node.

[feross]

The native crypto module is already used in node. git-sha1 handles that case. See: https://github.com/creationix/git-sha1/blob/master/git-sha1.js#L11

I don't mind moving to simple-sha1, but first we need to confirm:

• simple-sha1 (which uses Rusha internally) is actually faster than the git-sha1 implementation
• the file size difference when browserified is negligible
• swap out git-sha1 in all the webtorrent modules as it's used in several, and we don't want to include multiple implementations (I can handle this one)

[feross]

Confirmed all 3 things. Thanks for the PR!