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/builtins.cpp b/init/builtins.cpp
index ceab568..b6e26a1 100644
--- a/init/builtins.cpp
+++ b/init/builtins.cpp
@@ -1152,10 +1152,10 @@
 }
 
 // Builtin-function-map start
-const BuiltinFunctionMap::Map& BuiltinFunctionMap::map() const {
+const BuiltinFunctionMap& GetBuiltinFunctionMap() {
     constexpr std::size_t kMax = std::numeric_limits<std::size_t>::max();
     // clang-format off
-    static const Map builtin_functions = {
+    static const BuiltinFunctionMap builtin_functions = {
         {"bootchart",               {1,     1,    {false,  do_bootchart}}},
         {"chmod",                   {2,     2,    {true,   do_chmod}}},
         {"chown",                   {2,     3,    {true,   do_chown}}},