Inplace reduction functions

Inplace reduction functions (including sum!, prod!, maximum!, minimum!, all!, and any!) have been within the Base for a while. This PR exports them with a modified interface that is safe & flexible.
add documentation for sum! and friends
add an entry to NEWS.md about the reduction functions that allow pre-allocated output
add the link to 6197 in news
replace unbox warning with an assertion

this code is fine and the warning is not needed.
Merge pull request #6197 from lindahua/dh/inplacesum

Reduction functions with pre-allocated output storage.