Improve pointer hygeine in SocketAddresses.swift

[Lukasa]

Motivation:

The socket address types are some of the worst types for handling in
Swift. They pervasively type-pun in a way that Swift strongly wants to
resist, and they expose a bunch of data through awkward fixed-length
arrays.

We've done a number of passes over this code to improve its pointer
hygeine. This is another one. Here, we remove some uses of the memory
binding APIs that were unnecessary, and that can instead take advantage
of the way Swift handles binding memory of homogeneous tuple types.

In Swift, whenever we have a homogeneous tuple (i.e. a tuple whose
elements are all the same type), that memory is actually bound to two
types: to the tuple type, and to the type of the elements. This allows
us to use assumingMemoryBound(to:) to convert between the pointer to
the tuple and the pointer to the first element of the tuple.

This knowledge lets us remove calls to bindMemory and
withMemoryRebound. Neither call is correct to use in this situation,
and while we were unlikely to encounter any damage, this is the best
possible use of the Swift pointer APIs for this work.

Modifications:

• Rewrite accesses to the UNIX domain socket path to use appropriately
  typed pointers without binding or rebinding memory.
• Added some assertions about the static types of the stored elements.

Result:

Better management of pointer type bindings.

[glbrntt]

Looks good. Thanks for useful motivation + comments!

[PeterAdams-A]

Yes - agree this is an improvement over previous.