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.

test/compare_output.zig

@@ -122,7 +122,7 @@ pub fn addCases(cases: *tests.CompareOutputContext) void {
         \\
         \\pub fn main() void {
         \\    const stdout = &(io.getStdOut() catch unreachable).outStream().stream;
-        \\    stdout.print("Hello, world!\n{d4} {x3} {c}\n", u32(12), u16(0x12), u8('a')) catch unreachable;
+        \\    stdout.print("Hello, world!\n{d:4} {x:3} {c}\n", u32(12), u16(0x12), u8('a')) catch unreachable;
         \\}
     , "Hello, world!\n0012 012 a\n");