Improve microbench

- remove import statements (invoke with qjs --std)
- fix compatibility issues with node
- add more loop styles
- add more string concatenation tests
- use === and !=== where appropriate
- fix sort timing log
- add `-r ref_file` command line option to specify reference file
- add .gitignore file with target file patterns

.gitignore

@@ -0,0 +1,18 @@
+*.a
+.obj/
+examples/hello
+examples/hello_module
+hello.c
+microbench*.txt
+qjs
+qjsc
+qjscalc
+qjscalc.c
+repl.c
+run-test262
+test262
+test262_*.txt
+test262o
+test262o_*.txt
+unicode
+unicode_gen