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).

src/Generator/DefaultTimeGenerator.php

@@ -22,6 +22,17 @@ use Ramsey\Uuid\Provider\NodeProviderInterface;
 use Ramsey\Uuid\Provider\TimeProviderInterface;
 use Throwable;
 
+use function ctype_xdigit;
+use function dechex;
+use function hex2bin;
+use function is_int;
+use function pack;
+use function random_int;
+use function str_pad;
+use function strlen;
+
+use const STR_PAD_LEFT;
+
 /**
  * DefaultTimeGenerator generates strings of binary data based on a node ID,
  * clock sequence, and the current time