Run Psalm on src as well as Static Analysis Fixture (#282)

* Run Psalm on src as well as Static Analysis Fixture

Psalm baselining feature is used to prevent immediate build failures -
psalm will just cause failures for errors caused by future changes to
the code.

See https://psalm.dev/docs/running_psalm/dealing_with_code_issues/#using-a-baseline-file

* Fix MixedArgument issues in OrderedTimeCodec

Potentially unpack should be documented as returning string[] instead of
array to make this unecassary.

* Fix Psalm PossiblyNullPropertyAssignmentValue issues

* Add psalm-immutable annotation to Uuid class

* Fix Psalm MixedArgument issues

* Fix MissingImmutableAnnotation psalm issue

* Fix Psalm MixedAssignment issue

* Fix Psalm InvalidReturnStatement / InvalidReturnType issue

* Add ImpureMethodCall issues to baseline.

Fixing these in one place just moves them somewhere else.

* Exclude psalm-baseline.xml from exported git archives

src/Codec/OrderedTimeCodec.php

@@ -75,7 +75,11 @@ class OrderedTimeCodec extends StringCodec
             );
         }
 
-        $hex = unpack('H*', $bytes)[1];
+        $unpacked = unpack('H*', $bytes);
+
+        assert(is_string($unpacked[1]));
+
+        $hex = $unpacked[1];
 
         // Rearrange the fields to their original order
         $hex = substr($hex, 8, 4)