func: copy function definition in func_new

We need to duplicate a function for handling space upgrade in read view.
We can't just use func_new(func->def) to do this, because func_new sets
the given func_def to func->def, without copying. Usually, foo_new
duplicates the provided foo_def, e.g. see space_new. Let's make func_new
do the same.

Needed for https://github.com/tarantool/tarantool-ee/issues/163

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal