init: simplify keyword_map

I've heard that keyword_map is too complex, in particular the tuple
and the pair in BuiltinFunctionMap, so this change removes a lot of
that complexity and, more importantly, better documents how all of
this works.

Test: boot, init unit tests

Change-Id: I74e5f9de7f2ec524cb6127bb9da2956b5f307f56
diff --git a/init/action.h b/init/action.h
index 13b250a..80c1da4 100644
--- a/init/action.h
+++ b/init/action.h
@@ -75,7 +75,7 @@
     bool oneshot() const { return oneshot_; }
     const std::string& filename() const { return filename_; }
     int line() const { return line_; }
-    static void set_function_map(const KeywordFunctionMap* function_map) {
+    static void set_function_map(const BuiltinFunctionMap* function_map) {
         function_map_ = function_map;
     }
 
@@ -91,7 +91,7 @@
     Subcontext* subcontext_;
     std::string filename_;
     int line_;
-    static const KeywordFunctionMap* function_map_;
+    static const BuiltinFunctionMap* function_map_;
 };
 
 }  // namespace init