FIX - Re-add openssl

[Elrendio]

What it does

• Re-adds the openssl implementation with session re-use
• Keeps native-tls feature gated
• Adds From impl for FtpError to use ? in ftp.rs
• Resolves No way to implement ssl session reuse since #92 #96

@ChristianVisintin you might be interested 🙂
@mattnenterprise I guess you have a notification as maintainer but in case ^^

Notes on changes

Dependencies

I've removed the minor and patch versions in dependencies as that's equivalent to the carret condition which is what cargo uses by default (see cargo documentation). I believe it's best not to specify minor versions if not used with = (aka my_lib = "=1.2.3") as less experienced developers might not know that cargo can choose an newer version then what's written.

Imports

I've grouped dependencies (which I think should be the good practice) because it reduces the number of lines used for imports and allows a more visual representation of what it used in a library.

Welcome message

I was not convinced with the current implementation of the welcome message for two reason:

• It increases the memory size of the struct FtpStream for data you probably need to read only once
• The field welcome_msg is an option in the the struct FtpSteam suggesting it might not be here, however implementation made it available in all cases only to simplify the implementation of the function connect

Therefore I've made it available in the return of the function connect 🙂

Result vs crate::Result

I find it quite misleading to override import from rust prelude and notably for the result enum. If therefore moved the type Result<T> to lib.rs and use crate::Result to indicate the functions return a classical result of the crate rather than juste a classical result

About the put command

When uploading files to an ftp server using openssl I had the error FTP 426 Failure reading network stream.. My guess was that the ssl stream wasn't closed properly and that the master connection couldn't read until the data stream was closed. This is probably the case only with session re-use as without master and data connection are completly independent. Anyway, the fix consists in forcing the connection to shutdown after uploading the file. I'm quite unhappy of how I'm doing that, ideally it should have been in something equivalent to a Drop implementation. I didn't do a custom Drop implementation because I didn't know how to handle the possible connectivity errors when doing the shutdown. This is something to keep in mind for #93. Anyway, currently it works 🎉

[Elrendio]

@mattnenterprise When do you think you'll be able to merge this ? Thanks 🙂