Speed up compilation of overload.pm a smidge.

Speed up compilation of overload.pm a smidge.

Measured with the following crude perl script calling perf. Perl
is in there to get a rough baseline cost of starting perl:

 print 'PERL', (`perf stat -r100 perl -e 1             2>&1`)[10];
 print 'OLD ', (`perf stat -r100 perl lib/overload.pm  2>&1`)[10];
 print 'NEW ', (`perf stat -r100 perl lib/overload2.pm 2>&1`)[10];

Produced the following results on my machine:

 PERL  5,800,051 instructions # 1.05 insns per cycle ( +- 0.06% )
 OLD  14,818,995 instructions # 1.16 insns per cycle ( +- 0.03% )
 NEW  14,696,974 instructions # 1.16 insns per cycle ( +- 0.03% )

While the numbers did fluctuate between runs, the new code was
consistently faster.