Implement emplace, emplace_or_[c]visit

test/Jamfile.v2

@@ -215,9 +215,11 @@ rule build_cfoa ( name )
 build_cfoa insert_tests ;
 build_cfoa erase_tests ;
 build_cfoa try_emplace_tests ;
+build_cfoa emplace_tests ;
 
 alias cfoa_tests :
   cfoa_insert_tests
   cfoa_erase_tests
   cfoa_try_emplace_tests
+  cfoa_emplace_tests
 ;