Avoid creating single-element Perl arrays when the rule body unambiguously contains only one capturing subrule

[agentzh]

Often, a rule's production contains only one capturing subrule, like this:

foo: / a / bar / b /

Here only bar is a capturing subrule while neither / a / nor / b / do.

Hopefully the pegex engine can recognize such productions at compile time and avoid always creating a single-element Perl array for every got_foo() call. This way we can not only save some runtime overhead and can also save customized got_foo function definitions in the user receiver class. For example, I've found myself keep writing quite a lot of boring stubs for such productions like below:

sub got_foo() {
    my ($self, $list) = @_;
    $list->[0];
}

[ingydotnet]

@agentzh, can you please finish this out by providing a rule for bar and some
parsable input. Again if you can use this one-liner form, it is most helpful
for reproduction:

perl -MXXX -MPegex -e 'XXX pegex($grammar)->parse($input)'

[ingydotnet]

OK. I think I get it.

Basically:

a: b c d

is:

a: (b c d)1

Since there is a group, it adds the array.

Pegex isn't smart enough yet to know when an array has captures.

There's a ton of permutations and optimizations to consider. I'll start writing
a test suite for this tomorrow.

[agentzh]

@ingydotnet Great!