Getting rid of conversion from ‘long unsigned int’ to ‘unsigned int' may change value-messages

[medithe]

When compiling with -Wconversion in gcc 8, I'm getting the following warning:

error: conversion from ‘long unsigned int’ to ‘unsigned int’ may change value [-Werror=conversion]

This warning can be removed relatively simply by the following commit.

Noote, this is a follow up from #24, with a better solution.

[martinmoene]

Thanks @medithe,

I think for broadest support it should be size_t not std::size_t, see e.g. here. Note that this is different from in_place_index_tag which can assume size_t in namespace std.

Perhaps I should have written this more explicitly.

[medithe]

Ok, I agree, I have changed the pull request to use size_t without any namespace.
Thanks for your fast response.