Just some rearranging.

- Change used_return_value to return_value_used

- Initial support for Thies' idea of passing functions a flag telling them
they don't need to return a value. Adjusted reset() to work with this.
It is a bit problematic because if intern

- Initial support for Thies' idea of passing functions a flag telling them
they don't need to return a value. Adjusted reset() to work with this.
It is a bit problematic because if internal functions don't look at the
flag and still return a value orginally this leaked. So I free the value
in any case, which kind of makes the speed gain a bit smaller

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(PHP k[r]sort) now use compare_function from libzend to be consistent with the language-core.

Split array functions into separate module.