Support `-Dwithout_openssl`

[forchid]

When not use the openssl with compiler flag -Dwithout_openssl, this lib can't be complied for depending on openssl. In crystal-mysql, ssl socket is optional, and the required digest SHA1 can be implemented by similar src\crystal\digest\sha1.cr in crystal-lang project.

[bcardiff]

That compiler's flag is not intended as a feature for packages in general. It's a need due to the compiler having an http in the playground.

What is the use case for having this library be able to be compiled with openssl?