compiler: Implement interface calls

This is a big combined change. Other changes in this commit:

  * Analyze makeinterface and make sure type switches don't include
    unnecessary cases.
  * Do not include CGo wrapper functions in the analyzer callgraph.
    This also avoids some unnecessary type IDs.
  * Give all Go named structs a name in LLVM.
  * Use such a named struct for compiler-generated task data.
  * Use the type and function names defined by the ssa and types
    package instead of generating our own.
  * Some improvements to function pointers.
  * A few other minor improvements.

The one thing lacking here is interface-to-interface assertions.

README.markdown

@@ -72,3 +72,6 @@ Implemented analysis passes:
     sleep, chan send, etc. It's parents are also blocking.
   * Check whether the scheduler is needed. It is only needed when there are `go`
     statements for blocking functions.
+  * Check whether a given type switch or type assert is possible with
+    [type-based alias analysis](https://en.wikipedia.org/wiki/Alias_analysis#Type-based_alias_analysis).
+    I would like to use flow-based alias analysis in the future.