Add export{} syntax to encapsulate things that should be abstract

Author: catamorphism

src/comp/metadata/cstore.rs

@@ -6,7 +6,7 @@ import std::map;
 import syntax::ast;
 import util::common::*;
 
-export cstore;
+export cstore::{};
 export cnum_map;
 export crate_metadata;
 export mk_cstore;

src/libcore/comm.rs

@@ -26,8 +26,8 @@ import task;
 
 export send;
 export recv;
-export chan;
-export port;
+export chan::{};
+export port::{};
 
 #[abi = "cdecl"]
 native mod rustrt {

src/libcore/ctypes.rs

@@ -8,7 +8,7 @@ which are not obviously correct for all potential platforms.
 export c_int, c_uint, long, longlong, unsigned, ulong, ulonglong;
 export intptr_t, uintptr_t;
 export uint32_t;
-export void;
+export void::{};
 export c_float, c_double;
 export size_t, ssize_t;
 export off_t, fd_t, pid_t;