Remove unneeded 'fixed' statements from SocketAddress

[GrabYourPitchforks]

Part of the "reduce unsafe" effort across the dotnet org.

A prototype static analysis rule identified SocketAddress as being a good candidate to remove a fixed statement. We can't entirely remove unsafe from the target method because the value passed in is a pointer, but at least we can remove the unsafe code surrounding the buffer access.

[GrabYourPitchforks]

@jkotas @BrennanConroy So this was an interesting little bit of refactoring, but the upshot is:

Don't hide the fact that we fundamentally have to interpret a pointer under the covers, since we need to reinterpret_cast<> the pointer anyway. Embrace this, but keep it simple, and then immediately delegate to safer workhorse routines.

So this means the only unsafe code is this:

return pSockaddr->sa_family switch
{
    ADDRESS_FAMILY.AF_INET => new SocketAddressIPv4(*(SOCKADDR_IN*)pSockaddr),
    ADDRESS_FAMILY.AF_INET6 => new SocketAddressIPv6(*(SOCKADDR_IN6*)pSockaddr),
    _ => null
};

The SOCKADDR_IN* struct definitions are generated by CsWin32 and are safe. (Well, safe enough for our use, since we don't try to be clever with them. The definitions will be even safer once CsWin32 gets support for inline arrays.)

[BrennanConroy]

IPAddress does accept a scopeId, any reason not to just pass it in?

[GrabYourPitchforks]

TBH I don't know. The original code didn't pass it in, and I tried to avoid changing the observable behavior in this PR. (The "TODO" comment is copied from the original code.)