NodeWalker and removal of reduce()

Summary:
A general solution is needed for walking these AST's and applying a transformation to them. This implements a typical visitor class from which you can extend to subscribe to nodes you want to handle. There's built-in methods for manipulating the tree as you go, and getting data from your children.

Test Plan:
Built a few walkers for another application, adapting this API as I found use cases.

Reviewed By: epriestley

Makefile

@@ -55,8 +55,9 @@ parser.yacc.o: parser.lex.hpp
 parser.lex.o: parser.yacc.hpp
 parser.o: parser.yacc.hpp
 node.o: parser.yacc.hpp
+walker.o: node.hpp walker.hpp
 
-libfbjs.a: parser.yacc.o parser.lex.o parser.o node.o dmg_fp_dtoa.o dmg_fp_g_fmt.o
+libfbjs.a: parser.yacc.o parser.lex.o parser.o node.o walker.o dmg_fp_dtoa.o dmg_fp_g_fmt.o
 	$(AR) rc $@ $^
 	$(AR) -s $@
 

TARGETS

@@ -4,6 +4,7 @@ cpp_library(
           'parser.yy',
           'node.cpp',
           'parser.cpp',
+          'walker.cpp',
          ],
   deps = [ ':libfbjs_support' ],
 )