Add methods to streams.

This makes streams polymorphic with files in many cases.
printf family functions still need porting.

examples/tcpclient.janet

@@ -1,7 +1,6 @@
-(def conn (net/connect "127.0.0.1" "8000"))
-(printf "Connected to %q!" conn)
-(net/write conn "Echo...")
-(print "Wrote to connection...")
-(def res (net/read conn 1024))
-(pp res)
-(net/close conn)
+(with [conn (net/connect "127.0.0.1" "8000")]
+  (printf "Connected to %q!" conn)
+  (:write conn "Echo...")
+  (print "Wrote to connection...")
+  (def res (:read conn 1024))
+  (pp res))

examples/tcpserver.janet

@@ -1,13 +1,13 @@
 (defn handler
   "Simple handler for connections."
   [stream]
-  (def id (gensym))
-  (def b @"")
-  (print "Connection " id "!")
-  (while (net/read stream 1024 b)
-    (net/write stream b)
-    (buffer/clear b))
-  (printf "Done %v!" id)
-  (net/close stream))
+  (defer (:close stream)
+    (def id (gensym))
+    (def b @"")
+    (print "Connection " id "!")
+    (while (:read stream 1024 b)
+      (:write stream b)
+      (buffer/clear b))
+    (printf "Done %v!" id)))
 
 (net/server "127.0.0.1" "8000" handler)