Use luaossl instead of luacrypto.

[saucisson]

The luacrypto module does not seem to be maintained anymore (mkottman/luacrypto#39). Its current version on luarocks does not accept Lua 5.3, which is annoying.
This pull request is a proposal to use luaossl instead, as it is compatible with newer version of Lua.

Warning: i am a totally newbie in openssl, so the proposed fixes might be totally wrong. I just checked that all tests still pass. Moreover, the luaossl module can throw errors. I did not add code to catch them.

[DorianGray]

We need to catch the errors at least on decode and return them as nil, message to match the rest of the behavior(to avoid users from having to add pcalls everywhere). More tests on this behavior would be ideal.

[saucisson]

From luaossl documentation, neither hmac.* functions nor digest.* ones throw errors.
But the pkey.* functions can throw. They are now wrapped within pcalls, and two tests have been added to check this behavior. Note that there is still an error thrown when the signature cannot be generated.

[DorianGray]

Awesome, I'm going to test this with a few of my projects to make sure it doesn't fail in weird ways and then merge. This looks great, thanks!

[saucisson]

It can cause problems to you and other people using pkeys instead of text keys, as they are expected to be instances of openssl.key in this branch, and crypto.pkey in master.
An easy to fix this is to use crypto to dump keys and give them back to openssl. It is implemented in the latest commits, and tests using luacrypto keys have been added back.

[DorianGray]

Hmm, if we're going to break compatibility anyways, we should make the interface accept string keys instead of pkey objects. Hide the implementation details from the user. I'll do that unless you get there first.

[saucisson]

I have updated the code and tests to only accept strings for keys. An assertion throws an error in case a pkey (from luacrypto or luaossl) is used, as it is an invalid use of the library.

[DorianGray]

Thanks! That looks really good.