Create a standalone tls library that does not require patching the Go standard library

[kriskwiatkowski]

Let's make it separated library instead of being part of Go standard library.
If possible we should keep mechanism allowing to swap sources of tris with Go implementation

[Lekensteyn]

Just for future readers, patching the standard Go library will still be needed until at least Go 1.12 depending on the use case. If you only need a TLS implementation with raw sockets, a standalone package might work after #169.

If you need support for HTTP2 using net/http, then it won't work because that package explicitly checks for an interface from crypto/tls.

With Go modules (which is enabled by default in 1.13?) it might be possible to replace certain libraries, but I have not fully investigated that possibility.