This reverts commit e269c16c.

This issue is fixed in slevomat/coding-standard 6.1.4. See thread at
https://github.com/slevomat/coding-standard/issues/883

While the rule is now disabled, it still is very much valuable to import most pure functions from the core
PHP scope, allowing us some marginal gains on anything that can be inlined by the engine (now or in future).

The rule does not allow selective exclusions, so we will need to keep it off for now.

Import used functions and constants via `use` statement to disallow ambiguity for compiler/static-analysis

As documented in https://wiki.php.net/rfc/use_global_elements, the engine (by default) does a local namespace
lookup, then falls back to global namespace when first calling a global function referenced in namespaced code,
unless that function is referenced via fully qualified name (FQN).

By using the FQN, the actual symbol can be looked up at compile-time, both by the PHP engine and by static analysis
tooling, allowing for compiler (in particular) optimizations to replace known hot-path functions with specialized opcodes.

Sadly, no actual benchmark at hand: the improvements can be minimal or massive, depending on where this library is
used (tight loops being most relevant).

Fixes #28

[ci skip]