Adding a memory limit option

[gazugafan]

Asynchronous processes currently spawn with PHP's default memory limit (whatever's configured in php.ini). In my case, I increase this limit as needed in certain scripts using ini_set. This lead to a problem where these scripts would spawn processes that run out of memory... even though the memory limit was increased in the script, the sub-processes would all get the smaller default memory limit from php.ini and silently crash. Not ideal and tough to debug!

This PR adds a memoryLimit method to the Pool class that lets you configure a memory limit for each process spawned by the pool. You can specify a specific memory limit (using bytes, KB, M, etc.). Or, if you leave the parameter out, it will calculate how much memory is remaining in the parent script and divide it by the concurrency value--spreading out the remaining memory to the number of processes expected to be running. If you don't call the memoryLimit method at all, nothing happens--making this feature backwards-compatible.

[gazugafan]

Ah, the failing tests for PHP 7.4 are probably due to the mixed type params used in the memoryLimit and bytes methods. Probably best to just remove the types for PHP 7 compatibility?

[MarcHagen]

My to cents, maybe not for this PR, but consider dropping PHP 7.4 support and moving the package more towards stricter typing.
#223 already made the start for this.
Adding return types to more functions will further enhance static analysis and less unpredictable behaviours.
(Same argument and reasoning as in #175)

[Nielsvanpach]

We're okay with dropping everything below 8.2 and tagging a new major release.
Feel free to submit a PR for this!