Benchmark results

Not really an *issue*, but thought I'd send you these.

Benchmark source

```
use v5.16;
use warnings;
use Benchmark 'cmpthese';
use Test::More;

package SMT::test {
use Sub::Multi::Tiny qw( $foo $bar );
sub first :M($foo, $bar) {
return $foo ** $bar;
}
sub second :M($foo) {
return $foo + 42;
}
}

package M {
use Kavorka qw(multi fun);

multi fun test ($foo, $bar) {
return $foo ** $bar;
}
multi fun test ($foo) {
return $foo + 42;
}
}

is(M::test(8,2), 64);
is(M::test(3,3), 27);
is(M::test(58), 100);
is(SMT::test(8,2), 64);
is(SMT::test(3,3), 27);
is(SMT::test(58), 100);

cmpthese -1, {
M => q{ M::test(3,3); SMT::test(0); },
SMT => q{ SMT::test(3,3); SMT::test(0); },
};
```

Results on my laptop:

```
ok 1
ok 2
ok 3
ok 4
ok 5
ok 6
Rate SMT M
SMT 255999/s -- -11%
M 286959/s 12% --
```

I had expected a bit of a slow down from your use of guards and global variables, but it actually performs at a pretty decent rate.

As you use a stringy eval to build your dispatcher, you could probably speed things up a little by avoiding the hashtable arity lookup thing at run time, and putting `if(@_==1) {...} elsif(@_==2) {...}` into the evaluated string itself. The only advantage in keeping the hash table lookup would be if you wanted to be able to insert new entries into it at run-time, but right now the hash is a lexical variable, so short of some PadWalker tricks, can't be altered by runtime code anyway.