Add new 'use function' sequence for importing namespaced functions

[igorw]

This is specified as the use_function RFC:

• https://wiki.php.net/rfc/use_function

[jippi]

❤️

[weltling]

The functionality looks good so far and the tests on windows and linux pass. Prosit!

[nikic]

nit s/use function/use const

[nikic]

Patch looks pretty good. Some comments:

• Constants are not always case-insensitive (actually they are case-sensitive by default), so just doing lcs without checking CONST_CS will not work correctly. Maybe use zend_get_constant to do the lookups? It also handles special const names, but I don't think those are important in this context.
• The filename for functions is stored in their op_array, so there is no need to track it in an additional HT.
• Because the function filenames are tracked in a separate HT and not via the op_array they will be always added, even if registration of the function is delayed to runtime. E.g. the following code throws an error, whereas the same code using classes does not:

<?php

if (0) {
    function foo() {

    }
}

use function bar\foo;

// Fatal error: Cannot use function bar\foo as foo because the name is already in use in /home/nikic/dev/php-src/t59.php on line 9

Edit: Don't know whether the last point is actually important, just noticed that it behaves differently from class uses ;)

[igorw]

TODOs:

• Replace the function_filenames hash-table with a lookup to the op_array. Zend/tests/use_function/conditional_function_declaration.phpt should pass after that.
• Figure out why Zend/tests/use_function/define_imported_before.phpt is failing on travis. It works locally.

Help on either of those would be appreciated.

[igorw]

Re-opening against PHP-5.6.