Feature: Add immutable/pure support for psalm

[tux-rampage]

Currently assertions cannot be used in pure/immutable contexts when running psalm.
This PR aims to add @psalm-pure annotations where applicable.

[tux-rampage]

Note: Build failing due to a regression in psalm (introduced in 3.8.2 - see vimeo/psalm#2649).
Fix available: #166

[tux-rampage]

@BackEndTea was the rejection of this PR on purpose?

[BackEndTea]

It was not 😅

[tux-rampage]

Ok, thanks. I'll rebase this branch later.

[Ocramius]

@tux-rampage can you give #160 (comment) a review? I'm gonna release it on a fork for now (need it on a project, not planning to publish), but I feel like we're duplicating a lot of work :S

[Ocramius]

Tag usable @ https://github.com/Ocramius/assert-1/releases/tag/1.7.99 meanwhile

{
    "...": "...",
    "require": {
        "webmozart/assert": "^1.7.99"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/Ocramius/assert-1",
            "comment": "Temporary fork to work around the missing release of https://github.com/webmozart/assert/pull/160"
        }
    ]
}

[tux-rampage]

Sure, I'll check back tomorrow.

[BackEndTea]

Thanks for your work @tux-rampage, but i've opted to merge #160 in favor of this PR.