Add md5 and sha1 template functions

[digitalcraftsman]

Fixes #1910.

[digitalcraftsman]

@bep since the template functions' name would conflict with the corresponding package names I added an _ as prefix. Let me know if you're not happy with the way how the conflict is handled.

[bep]

an _ as prefix.

That's fine. But you might add the error case to the tests (see the expect false interface{} construct used elsewhere).

Also add the two functions to the "smoke test at the top of the file".

[digitalcraftsman]

I updated the test file accordingly.

[bep]

Merged in 94c3825

[digitalcraftsman]

Thanks.

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