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).

tests/Generator/RandomBytesGeneratorTest.php

@@ -10,6 +10,8 @@ use Ramsey\Uuid\Exception\RandomSourceException;
 use Ramsey\Uuid\Generator\RandomBytesGenerator;
 use Ramsey\Uuid\Test\TestCase;
 
+use function hex2bin;
+
 class RandomBytesGeneratorTest extends TestCase
 {
     /**