Make sum work with const slices

[jmh530]

Addresses issue #454.

Hopefully checked enough that it works. As is, changes to get working are relatively minimal. Ideally the compiler should be able to optimize away the removal of moves for the const/immutable case.

[9il]

Please add additional overload. One for const, one for mutable. The const overload has to call lightConst and then call the mutable implementation.

[jmh530]

@9il Can you remind me what the significance of lightConst is?

[9il]

It moves const from slice type to member type.

[jmh530]

@9il Thanks, see latest commit. It's a little tricky in that core.lifetime.move only works properly when you assign the lightConst to a new variable.

I also need to add UT examples in Slice to really remember on how lightConst works.

[9il]

We will not do that for other functions. A user has to call .lightConst in the code instead.

[jmh530]

We will not do that for other functions. A user has to call .lightConst in the code instead.

When you refer to "that", I'm not sure what you mean. Could you help me understand what you mean?

[9il]

I mean the const slice support.