feat: Adds parameter typehints to ContainerInterface

This patch bumps the minimum supported PHP version to 7.2 and adds
parameter typehints to ContainerInterface, as the first step towards
adding explicit typehints based on the specification.

See https://www.php-fig.org/blog/2019/10/upgrading-psr-interfaces/

composer.json

@@ -12,7 +12,7 @@
         }
     ],
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=7.2.0"
     },
     "autoload": {
         "psr-4": {
@@ -21,7 +21,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "1.0.x-dev"
+            "dev-master": "2.0.x-dev"
         }
     }
 }

src/ContainerInterface.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Psr\Container;
 
 /**
@@ -17,7 +19,7 @@ interface ContainerInterface
      *
      * @return mixed Entry.
      */
-    public function get($id);
+    public function get(string $id);
 
     /**
      * Returns true if the container can return an entry for the given identifier.
@@ -30,5 +32,5 @@ public function get($id);
      *
      * @return bool
      */
-    public function has($id);
+    public function has(string $id);
 }