Introduce function reference value types

[veewee]

See #5

This PR introduces support for function reference value types.

For example:

$builder = InstanceBuilder::fromWat(
    <<<'EOWAT'
    (module
        (import "env" "greet" (func $greet))
        (func
            call $greet
        )
        (start 1) ;; run the first function automatically
    )
    EOWAT
);

$imports = Imports::create();
$imports->define('env', 'greet', Type\Global::immutable(
    function () use ($result) {
        echo 'Hello!';
    }
));


$builder->import($imports);
$instance = $builder->build();

Hello!