* wip
* move print and JSON functions to modules
* builtin functions are not replacable now
* builtin functions are added for default nil symbol table
* importables: builtin modules and source modules
* refactoring runtime tests
* fix tests
* update documentation
* cleanup
* clean up cli
* fix REPL prints
- add object.NumObjects()
- add object allocation limit in VM
- delete objects.Break, objects.Continue, objects.ReturnValue
- add Script.SetMaxAllocs()
- update sandbox documentation
- add some tests
- remove duplicate values in compiled constants (fixes#96)
- option to limit the maximum number of objects in compiled bytecode constants
- add type infos to VM error messages
- add 'Name' to UserFunction objects
- add 'expectErrorString' to VM tests
- replace vm.expectError() with vm.expectErrorString() to make it more explicit
- add source map info to VM error messages
- optimization in function calls
- add file/line/col info to compiler errors
- change stdlib module to be loaded from VM (instead of compiler) so they can be properly loaded after the source is compiled into binary
- VM can take builtin modules optionally
