utils::in: Add helper type for perfect forwarding initializer lists
utils::in: Add helper type for perfect forwarding initializer lists

wrapper type (courtesy of
http://cpptruths.blogspot.se/2013/09/21-ways-of-passing-parameters-plus-one.html#inTidiom)
to enable move semantics in initializer lists. Useful as an engineering
overkill to retain nice call sites. 
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