Add crypto module

[bartlomieju]

SHA1 is required for #29

[hayd]

The best long-term solution is to get this by including webcrypto via gn in main deno.

[dsseng]

I'll try to make simple sha1 hash function in TS.

[hayd]

See e.g. https://github.com/keroxp/deno-ws/blob/master/crypt.ts

[ry]

Probably @hayd is correct that webcrypto would be best (so this issue should be in deno instead of deno_std)
I have to investigate further - but it looks like one road block is to get //base working... We'd have to weigh the benefits of that against rolling our own. //base would introduce a lot of duplicated code that's already in Rust, for example.
In any case, there are several refactors that we need to complete before addressing this - in particular denoland/deno#975