Extracted functions to static methods

[GrahamCampbell]

This PR pairs with guzzle/guzzle#2712. After this is merged, we can tag 1.4.0.

TODO:

• Extract all functions to static class methods
• Deprecate all functions and point to their replacements
• Replace all uses in src
• Update the tests
• Update documentation and examples
• Update change log

Mappings:

Original Function	Replacement Method
queue	Utils::queue
task	Utils::task
promise_for	Create::promiseFor
rejection_for	Create::rejectionFor
exception_for	Create::exceptionFor
iter_for	Create::iterFor
inspect	Utils::inspect
inspect_all	Utils::inspectAll
unwrap	Utils::unwrap
all	Utils::all
some	Utils::some
any	Utils::any
settle	Utils::settle
each	Each::of
each_limit	Each::ofLimit
each_limit_all	Each::ofLimitAll
!is_fulfilled	Is::pending
is_fulfilled	Is::fulfilled
is_rejected	Is::rejected
is_settled	Is::settled
coroutine	Coroutine::of

NB The overall idea of these changes was approved by the original author, Michael: #108.

[GrahamCampbell]

Friendly ping @Tobion and @Nyholm. It'd be good to have this merged so I can focus on PHP 8.0 support next, without conflicts.

[Nyholm]

Thank you @GrahamCampbell for this.

I've reviewed it multiple times over the months and today I decided to be super picky and finish it all.

I found just one small thing that only "technically" could have a side effect. Good job!