Enforcing purity/immutability on most of the converter API

The API of this library is pretty much pure/referentially transparent,
and can therefore be used in contexts where I/O is to be avoided

.travis.yml

@@ -10,10 +10,19 @@ php:
 matrix:
   include:
     - php: 7.2
+      script:
+        - vendor/bin/psalm
+        - vendor/bin/phpunit
       env: dependencies=lowest
     - php: 7.3
+      script:
+        - vendor/bin/psalm
+        - vendor/bin/phpunit
       env: dependencies=lowest
     - php: 7.4
+      script:
+        - vendor/bin/psalm
+        - vendor/bin/phpunit
       env: dependencies=highest
 
 ## Update composer and run the appropriate composer command

composer.json

@@ -20,7 +20,8 @@
     "moontoast/math": "^1.1"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7.5.20"
+    "phpunit/phpunit": "^7.5.20",
+    "vimeo/psalm": "^3.10"
   },
   "suggest": {
     "ramsey/uuid": "A PHP 5.4+ library for generating RFC 4122 version 1, 3, 4, and 5 universally unique identifiers (UUID)"