sprintf for S29

I've taken a stab at sprintf for S29. All I've done differently from
Perl 5 is to drop %p and %n (generates an exception) and add %C
(closure) to replace the functionality of %n. Are there any additional
formats that we want to add?

Other than that, this should be exactly the same as Perl 5:

=item sprintf

 our method Str Str::sprintf ( Str $format: *@args )
 our multi Str sprintf ( Str $format, *@args )

This function is mostly identical to the C library sprintf function.

The C<$format> is scanned for C<%> characters. Any C<%> introduces a
format token. Format tokens have the following grammar:

 grammar Str::SprintfFormat {
  rule format_token { \%: <index>? <precision>? <modifier>? <directive> }
  rule index { \d+ \$ }
  rule precision { <flags>? <vector>? <precision_count> }
  rule flags { <[\ +0\#\-]>+ }
  rule precision_count { [ <[1-9]>\d* | \* ]? [ \. [ \d* | \* ] ]? }
  rule vector { \*? v }
  rule modifier { <[lhVqL]> | ll }
  rule directive { <[\%csduoxefgXEGbpniDUOF]> }
 }

Directives guide the use (if any) of the arguments. When a directive
(other than C<%>) are used, they indicate how the next argument
passed is to be formatted into the string.

The directives are:

 %   a percent sign
 c   a character with the given number
 s   a string
 d   a signed integer, in decimal
 u   an unsigned integer, in decimal
 o   an unsigned integer, in octal
 x   an unsigned integer, in hexadecimal
 e   a floating-point number, in scientific notation
 f   a floating-point number, in fixed decimal notation
 g   a floating-point number, in %e or %f notation
 X   like x, but using upper-case letters
 E   like e, but using an upper-case "E"
 G   like g, but with an upper-case "E" (if applicable)
 b   an unsigned integer, in binary
 C   special: invokes the arg as code, see below

Compatibility:

 i   a synonym for %d
 D   a synonym for %ld
 U   a synonym for %lu
 O   a synonym for %lo
 F   a synonym for %f

Perl 5 compatibility:

 n   produces a runtime exception (see below)
 p   produces a runtime exception

The special format directive, C<%C> invokes the target argument as
code, passing it the result string that has been generated thus
far and the argument array.

Here's an example of its use:

 sprintf "%d%C is %d digits long",
        $num,
        sub($s,@args is rw) {@args[2]=$s.size},
        0;

The special directive, C<%n> does not work in Perl 6 because of the
difference in parameter passing conventions, but the example above
simulates its effect using C<%C>.

=cut

-- 
Aaron Sherman <ajs@ajs.com>
Senior Systems Engineer and Toolsmith
"We had some good machines, but they don't work no more." -Shriekback

• sprintf for S29 by Aaron Sherman