fix: Migrate from amphp/parallel-functions to amphp/parallel

[theofidry]

In #723 I disabled the parallelization by default as building the Box PHAR with parallelization was x1000 slower than without.

Since then, a number of things changed in PHP-Scoper and in preparation to adapt the way the parallelization is done, a few things changed in Box too. I am not sure what element made a difference but according to PHPBench, enabling parallelization was reducing the build time from 13.932s to 9.243s so a ~33% speed improvement.

I however gave it more though I as thinking serializing closures is a bad idea of a source of too many problems. Currently using parallelization in Box results in errors due to data being readonly. I do not remember exactly in what part laravel/serializable-closure messes up but I also remembered it was a limitation of the library (and in fairness, the library is really not at fault, it tries really hard to hack its way through to patch a very annoying PHP limitation, so it does what it can).

But I kept finding those issues too annoying and I think the solution is also quite simple: to use AMPHP at a level lower which would avoid to have to serialize closures altogether.

Closes #552.
Closes #602 (the memory issue seems gone according to the PHPBench results).
Closes #1160 (superseded).

With those changes, we get the following results:

subject	memory	mode	rstdev	stdev
with PHP-Scoper; no parallel processing	940.391mb	13.932s	±0.88%	122.973ms
with PHP-Scoper; parallel processing	47.957mb	6.668s	±0.64%	42.899ms

In other words the parallelization goes from 9.243s to 6.668s, so a further ~28% improvement making it ~52% faster than without parallelization.