selfhost: for-in support for Channel<T>; fix break/continue C codegen

- lib/Channel.bux: add Channel_Recv_Ok<T> for use in for-in desugaring
- hir_lower.bux: Channel for-in desugars to while+recv_ok pattern
- c_backend.bux: emit 'break;' / 'continue;' with semicolon, fix missing
  semicolon when break/continue inside if-block
- bootstrap/hir_lower.nim: skip generic methods in vtable generation to
  avoid referencing non-existent monomorphized C functions

lib/Channel.bux

@@ -24,6 +24,10 @@ func Channel_Recv<T>(ch: *Channel<T>) -> T {
     return result;
 }
 
+func Channel_Recv_Ok<T>(ch: *Channel<T>, out: *T) -> bool {
+    return bux_channel_recv(ch.handle, out as *void) != 0;
+}
+
 func Channel_Close<T>(ch: *Channel<T>) {
     bux_channel_close(ch.handle);
 }