add ruby String to rust Byte Collection conversion

[cramt]

i Added support for converting a ruby String into a rust byte collection

In ruby the main way to represent some bytes is with a string, (see File.read or Base64.strict_decode64). and the current way to accept such
a string with magnus is by receiving a RString and calling .as\_slice().as\_vec() which is unsafe, this creates safe interface to receive bytes from ruby.

I've decided to use the Bytes crate since it seems to be rust communty's prefered way of dealing with collections of bytes and made the inclusion of it a non defaulted feature

[matsadler]

Thanks, this is great!

Could you add an IntoValue implementation for Bytes too? That way it can also be returned to Ruby.
I'm in the process of transitioning from Into<Value> to IntoValue, but Into<Value> won't go away til after the next release, so there should probably be a impl From<bytes::Bytes> for Value too.

I'll add an entry to the CHANGELOG.md for this after I merge, but feel free to add it there yourself in your own words if you want to.

[matsadler]

Thank you!