Change libspirv to spvtools namespace (#1678)

This CL changes all of the libspirv namespace code to spvtools to match
the rest of the code base.
diff --git a/source/enum_string_mapping.cpp b/source/enum_string_mapping.cpp
index e993b58..20bbdfc 100644
--- a/source/enum_string_mapping.cpp
+++ b/source/enum_string_mapping.cpp
@@ -22,8 +22,8 @@
 
 #include "extensions.h"
 
-namespace libspirv {
+namespace spvtools {
 
 #include "enum_string_mapping.inc"
 
-}  // namespace libspirv
+}  // namespace spvtools