std: FIXME stub net::ip::ip_addr::ipv6 variant...needs parse/format impl

still need implementation for parsing/output formatting and (perhaps?)
representation (for now, i just followef the ipv4 variant's lead and
am representing it as a tuple of 8x u16).

parsing an ipv6 addr is way more complex than parsing an ipv4 addr, so
i'm putting off an implementation here, for now.

candidate solutions:
- could use getaddrinfo() (exists on both POSIX and windows), but with
incompatible fn signatures.
- libuv has a way to parse an ipv6 string into
a sockaddr_in6, but it also requires a port, so it's probably not aprop
for ip_addr

src/libstd/net_ip.rs

@@ -14,6 +14,7 @@ export v4;
 enum ip_addr {
     #[doc="An IPv4 address"]
     ipv4(u8, u8, u8, u8),
+    ipv6(u16,u16,u16,u16,u16,u16,u16,u16)
 }
 
 #[doc="
@@ -28,6 +29,9 @@ fn format_addr(ip: ip_addr) -> str {
       ipv4(a, b, c, d) {
         #fmt["%u.%u.%u.%u", a as uint, b as uint, c as uint, d as uint]
       }
+      ipv6(a,b,c,d,e,f,g,h) {
+        fail "FIXME impl parsing of ipv6 addr";
+      }
     }
 }