Whitelist functions as soon as they are used

[theofidry]

Previously, if you had the following code:

namespace Acme;

if (function_exists('foo')) {
    \foo();
}

with foo being in the whitelist, the function would have not been whitelisted. The reasoning was that the recording of whitelisted elements was the same for functions and classes. For classes, it makes sense since the whitelisting will not work if the whitelisted class declaration is not found, however this is not the case for functions since the declaration dumped in the scoper-autoload.php is sufficient.

With this PR, the function foo above is now prefixed, which should allow cases like this one to work out of the box, either by whitelisting the global functions or by manually whitelisting it.