Don't try to case change surrogates, above-Unicodes

Don't try to case change surrogates, above-Unicodes

Changing the case (upper, lower, title, fold) of surrogate code points
and non-Unicode code points always yields the original, so there is no
need to actually try it.  And trying it is slow and creates swashes,
which uses up runtime memory.  We test for these code points anyway, so
at the cost of just two gotos and a label, we can skip all that work and
potential memory use.  This is worth doing even though such usage will
be rare in practice.

Running the following command

    blead Porting/bench.pl --perlargs="-Ilib -X" --benchfile=above_unicode path_to_prior_perl=before_this_commit path_to_this_perl=after

on a -O2 no DEBUGGING perl, where file 'above_unicode" contains

    [
        'string::casing::above_unicode' => {
            desc    => 'yes cases vs no casing',
            setup   => 'my $a = "\x{110000}"',
            code    => 'my $b = uc($a)'
        },
    ];

yields this output (the extra cost of swash creation is not included):

 The numbers represent raw counts per loop iteration.

 string::casing::above_unicode
 yes cases vs no casing

        before_this_commit    after
        ------------------ --------
     Ir             1329.0    651.0
     Dr              324.0    190.0
     Dw              149.0     94.0
   COND              192.0    103.0
    IND               13.0     10.0

 COND_m                5.5      0.0
  IND_m                6.0      4.0

  Ir_m1                0.1      0.0
  Dr_m1                0.0      0.0
  Dw_m1                0.0      0.0

  Ir_mm                0.0      0.0
  Dr_mm                0.0      0.0
  Dw_mm                0.0      0.0