breaking: Add positional, precision and width support to std.fmt

This removes the odd width and precision specifiers found and replacing
them with the more consistent api described in #1358.

Take the following example:

    {1:5.9}

This refers to the first argument (0-indexed) in the argument list. It
will be printed with a minimum width of 5 and will have a precision of 9
(if applicable).

Not all types correctly use these parameters just yet. There are still
some missing gaps to fill in. Fill characters and alignment have yet to
be implemented.

Author: tiehuis

src-self-hosted/dep_tokenizer.zig

@@ -999,7 +999,7 @@ fn printCharValues(out: var, bytes: []const u8) !void {
 
 fn printUnderstandableChar(out: var, char: u8) !void {
     if (!std.ascii.isPrint(char) or char == ' ') {
-        std.fmt.format(out.context, anyerror, out.output, "\\x{X2}", char) catch {};
+        std.fmt.format(out.context, anyerror, out.output, "\\x{X:2}", char) catch {};
     } else {
         try out.write("'");
         try out.write([_]u8{printable_char_tab[char]});