Split IArray into IStruct and ITable and introduce direction Table.

Adapted fromArray methods to now use struct and table instead of array.
Adapted all tests to pass with the new Table constructor parameter.

Author: gregor-j

composer.json

@@ -19,7 +19,7 @@
   "require": {
     "php": "^7.0",
     "ext-json": "*",
-    "php-sap/interfaces": "^2.0",
+    "php-sap/interfaces": "dev-master",
     "php-sap/datetime": "^1.2"
   },
   "autoload": {