Reduce memory consumption in FilesCollection

[ckuran]

Q	A
Branch	2.x
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Documented?	n/a
Fixed tickets	#1218

This PR reduces memory consumption in FilesCollection. The key changes:

1. Lower memory in parallel mode. When GrumPHP runs tasks in parallel via amphp/parallel, the file collection is serialized and shipped to worker processes. Previously each file was immediately unpacked into a full SymfonySplFileInfo object levereging a lot of memory. Now the worker receives plain strings and only builds the objects when something actually iterates over them
2. ensureFiles() - appends one file collection to another without duplicates. It now keeps known paths in a plain hash-map array, so duplicate checks are constant across time.
3. Added printing Peak memory usage: X.X MB at the end of commands for easier diagnostic (only in verbose mode)

Results

Before

Running tasks with priority 0!
==============================

Running task 1/12: jsonlint... ✔
Running task 2/12: xmllint... ✔
Running task 3/12: yamllint... ✔
Running task 4/12: openapi... ✔
Running task 5/12: rector... ✔
Running task 6/12: behat... ✔
Running task 7/12: phpmd... ✔
Running task 8/12: phpcs... ✔
Running task 9/12: phpstan... ✔
Running task 10/12: psalm... ✔
Running task 11/12: test-unit... ✔
Running task 12/12: test-integration... ✔

Peak memory usage: 1152.0 MB

After

Running tasks with priority 0!
==============================

Running task 1/12: jsonlint... ✔
Running task 2/12: xmllint... ✔
Running task 3/12: yamllint... ✔
Running task 4/12: openapi... ✔
Running task 5/12: rector... ✔
Running task 6/12: behat... ✔
Running task 7/12: phpmd... ✔
Running task 8/12: phpcs... ✔
Running task 9/12: phpstan... ✔
Running task 10/12: psalm... ✔
Running task 11/12: test-unit... ✔
Running task 12/12: test-integration... ✔

Peak memory usage: 126.0 MB