fix: Migrate from amphp/parallel-functions to amphp/parallel (#1216)

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Closes #1160 (superseded).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.**

Makefile

@@ -487,18 +487,19 @@ $(REQUIREMENT_CHECKER_EXTRACT):
 
 $(SCOPED_BOX_BIN): $(SCOPED_BOX_DEPS)
 	@echo "$(YELLOW_COLOR)Compile Box.$(NO_COLOR)"
-	$(BOX) compile --ansi --no-parallel
+	@# Use parallelization
+	$(BOX) compile --ansi
 
 	rm $(TMP_SCOPED_BOX_BIN) || true
 	mv -v bin/box.phar $(TMP_SCOPED_BOX_BIN)
 
 	@echo "$(YELLOW_COLOR)Compile Box with the isolated Box PHAR.$(NO_COLOR)"
-	php $(TMP_SCOPED_BOX_BIN) compile --ansi --no-parallel
+	php $(TMP_SCOPED_BOX_BIN) compile --ansi
 
 	mv -fv bin/box.phar box
 
 	@echo "$(YELLOW_COLOR)Test the PHAR which has been created by the isolated PHAR.$(NO_COLOR)"
-	./box compile --ansi --no-parallel
+	./box compile --ansi
 
 	mv -fv box bin/box.phar
 	rm $(TMP_SCOPED_BOX_BIN)

bin/bench-test.php

@@ -45,6 +45,7 @@
 
 if ($parallelTime <= $maxParallelTimeTarget) {
     echo 'OK.'.PHP_EOL;
+
     exit(0);
 }
 
@@ -59,5 +60,5 @@
 
 echo 'Failed!'.PHP_EOL;
 echo 'Missed the target by '.$relativeDifference.'%'.PHP_EOL;
-// TODO: https://github.com/box-project/box/issues/552
-exit(0);
+
+exit(1);

composer.json

@@ -22,7 +22,7 @@
         "ext-mbstring": "*",
         "ext-phar": "*",
         "composer-plugin-api": "^2.2",
-        "amphp/parallel-functions": "^1.1",
+        "amphp/parallel": "^2.0",
         "composer/semver": "^3.3.2",
         "composer/xdebug-handler": "^3.0.3",
         "fidry/console": "^0.6.0",