Children functions need to be hoisted and instantiated at the beginning
of a function call. Previously, it was done as a part of C code
that implements JS function call.

Now, each child function is instantiated by FUNCTION instruction at a
function prelude. This makes global and function code similar, which in
turn allows to get rid of FUNCTION COPY instruction which was only
needed for global code.

The issue was introduced in 6335367 (0.7.0).

This fixes nginx#530 issue on Github.