* v test: support for running 'v test folder/' .
* Support passing multiple folders and also single _test.v files to 'v test' .
* Update vhelp too, with descriptions of v test folder/ and v -stats .
* Fix running `v test v` from outside the root of the v tree.
* compiler: extract and cleanup error handling functionality into its own file compiler/compile_errors.v
* compiler: implement p.error_with_token_index and p.warn_with_token_index and use them. Fix tests.
* tools/performance_compare: add a 'Source lines in compiler/' line
* MSVC does not have STDOUT_FILENO nor STDERR_FILENO ...
