Change make_strings_children to return uvector

[davidwendt]

Description

Changes the cudf::strings::detail::make_strings_children utility to return a rmm::device_uvector<char> instead of a chars column. This further helps enable large strings support by not storing chars in a column.
This is an internal utility and so is non-breaking for any public APIs.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[PointKernel]

Looks great with a non-blocking comment on the function return type.

[PointKernel]

Preferably returning a unique pointer of uvector according to the developer guide,

cudf/cpp/doxygen/developer_guide/DEVELOPER_GUIDE.md

Lines 185 to 190 in 63e9040

	libcudf functions typically take views as input (`column_view` or `table_view`)
	and produce `unique_ptr`s to owning objects as output. For example,
	```c++
	std::unique_ptr<table> sort(table_view const& input);
	```

I also noticed #14202 changed the input argument from unique ptr to rvalue reference for make_strings_column, which makes the ownership information obscure IMO.

[PointKernel]

Suggested change

	return std::pair(std::move(offsets_column), std::move(chars));
	return std::pair(std::move(offsets_column), chars);

nit: no need to move if it's just a uvector

[davidwendt]

Without the std::move the uvector is copied.
https://godbolt.org/z/K1sTfP1na

[PointKernel]

I thought copy elision could happen automatically with C++17 based on multiple discussions at our cpp channel and also StackOverflow and then by taking a closer look at your sample code, I realized that returning a pair makes a difference (see here).

[davidwendt]

/merge