selfhost: add *_Drop wrappers to stdlib types; auto-drop uses _Drop instead of _Free

- Array.bux, Channel.bux, Set.bux, Map.bux: add *_Drop<T> delegating to *_Free
- hir_lower.bux: Lcx_BuildAutoDropFree searches for _Drop suffix instead of _Free
- Skip direct lowering of generic functions (only monomorphized instances)

lib/Channel.bux

@@ -32,6 +32,10 @@ func Channel_Free<T>(ch: *Channel<T>) {
     bux_channel_free(ch.handle);
 }
 
+func Channel_Drop<T>(ch: *Channel<T>) {
+    Channel_Free<T>(ch);
+}
+
 /* Convenience wrappers for common types */
 func Channel_SendInt(ch: *Channel<int>, value: int) {
     bux_channel_send(ch.handle, (&value) as *void);