Previously, the compiler had special logic to determine whether to
include the startup code, which was in `std/special/start.zig`. Now,
the file is moved to `std/start.zig`, and there is no special logic
in the compiler. Instead, the standard library unconditionally imports
the `start.zig` file, which then has a `comptime` block that does the
logic of determining what, if any, start symbols to export. Instead of
`start.zig` being in its own special package, it is just another normal
file that is part of the standard library.
`std.builtin.TestFn` is now part of the standard library rather than
specially generated by the compiler.
* use erase rest of line escape code.
* use `stderr.supportsAnsiEscapeCodes` rather than `isTty`.
* respect `--color off`
* avoid unnecessary recursion
* add `Progress.log`
* disable the progress std lib test since it's noisy and uses
`time.sleep()`.
* enable/integrate progress printing with the default test runner
