Context and socket helpers

I wrote some procedures that I have found helpful in my work.  I thought you might like them too:

``` text
(call-with-socket	 	context	 	 	 	 
 	 	socket-type	 	 	 	 
 	 	procedure)	 	→	 	void
  context : context?
  socket-type : socket-type?
  procedure : (procedure-arity-includes/c 1)
```

and

``` text
(call-with-context	 	procedure	 	 	 	 
 	 [	#:io-threads io-threads])	 	→	 	void
  procedure : (procedure-arity-includes/c 1)
  io-threads : exact-nonnegative-integer? = 1
```