Revamp `enable_if` constraints in all env overloads

[gonidelis]

It has been frequently discussed throughout the PRs that add environment overloads that we make our enable_if constrains more uniform to:

a) reduce pronity to unforeseen ambiguity errors
b) make code more readable
c) avoid potential compiler oversights

Μany enable_if constraints are unnecessarily complex. The parameter type differences alone (KeyT* vs void*, concrete iterator vs size_t&) are enough for disambiguation, the is_integral_v<NumItemsT> guard is redundant in most cases.

Instances of these discussions:

#7969 (comment)
#7999 (comment)
#7908 (comment)
#7794 (comment)
#5335 (comment)
#7970 (comment)
#7907 (comment)
#7965 (comment)
#7798 (comment)
#7967 (comment)

@bernhardmgruber and @miscco have been heavily involved in this discussions in many of their reviews.

Close this PR by verifying that all enable_if in all env overloads work as expected, create no ambiguities and use no buggy traits.

This issue should be closed, after all env PRs are merged.

[bernhardmgruber]

@miscco agreed to help us here as well!

[gonidelis]

@miscco agreed to help us here as well!

I always love me some Michael help