Feature/php8 support (#629)

Adds support for PHP8.0, dropping support for PHP7.0 and below.

Backwards incompatible changes:
- Dropped support for php asp tags in templates (removed from php since php7.0)
- Dropped deprecated API calls that where only accessible through SmartyBC
- Dropped support for {php} and {include_php} tags and embedded PHP in templates. Embedded PHP will now be passed through as is.
- Removed all PHP_VERSION_ID and compare_version checks and conditional code blocks that are now no longer required
- Dropped deprecated SMARTY_RESOURCE_CHAR_SET and SMARTY_RESOURCE_DATE_FORMAT constants
- Dropped deprecated Smarty::muteExpectedErrors and Smarty::unmuteExpectedErrors API methods
- Dropped deprecated $smarty->getVariable() method. Use $smarty->getTemplateVars() instead.
- $smarty->registerResource() no longer accepts an array of callback functions

See the changelog for more details.

Switched CI from Travis to Github CI.

tests/UnitTests/SmartyMethodsTests/ClearAllAssign/ClearAllAssignTest.php

@@ -18,7 +18,7 @@ class ClearAllAssignTest extends PHPUnit_Smarty
     protected $_data = null;
     protected $_tpl = null;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->setUpSmarty(dirname(__FILE__));