Nest SocketAddress in NIOHTTPServer

[aryan-25]

Motivation:

The HTTPServer module defines a SocketAddress type, but this has the same name as the type in the NIOCore module.

Due to a compiler bug, when a module imports both HTTPServer and NIOCore, the HTTPServer module's SocketAddress type cannot be referenced, because HTTPServer also contains a protocol with the same name (the HTTPServer protocol). In this case, using the module name as a namespace qualifier doesn't work as intended:

import HTTPServer
import NIOCore

let a: SocketAddress // error: 'SocketAddress' is ambiguous for type lookup in this context

let b: HTTPServer.SocketAddress // error: 'SocketAddress' is not a member type of protocol 'HTTPServer.HTTPServer'

let c: NIOCore.SocketAddress // compiles fine.

Modifications:

Nested SocketAddress under NIOHTTPServer, since that type is only used in the concrete NIO implementation.

Result:

NIOHTTPServer.SocketAddress can be used without running into swiftlang/swift#43510.