use php7.4 functionality

[arnedesmedt]

Hi,

I've used the new type hinting functionality for properties in PHP 7.4 to create more efficient ImmutableRecords.

But the only problem is that it would give conflicts if people still use older PHP versions. Maybe you can create a dev branch for the PHP 7.4 functionality or merge this branch in master and create a dev branch for people who want to use older PHP versions?

What are your taughts about the last problem?

[codeliner]

really nice. Thank you @arnedesmedt
I'd do the following:
It's time to tag a v1.0 version. The package is pretty stable and used a lot in different important projects (important for my company and our clients). The current master becomes v1.0.
After that, I'll merge your PR and create a v2.0-beta.1 for those who are on PHP7.4 and want to make use of prop types within ImmutableRecords. When v2.0-beta.x feels stable and projects switch to it, I'd turn it into v2.0.