During all "unwrap" functions, return the real values instead of values wrapped with Node\Arg() class

[maks-rafalko]

During all "unwrap" functions, return the real values instead of values wrapped with Node\Arg() class

Fixes #1799

Failing (before) code:

$array->{substr($key, 0, -2)}()

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Explanation: before this update, we incorrectly return all the values, wrapped with Node\Arg() class, since before mutating they were arguments of unwrapped function.

For example:

- $a = substr('test', 0, -2);
+ $a = 'test';

here, in pseudo-AST, we did the following:

$a = Node\Arg('test');

which is not correct