explore/static/utility.h: Fix build failure on 32bit
explore/static/utility.h: Fix build failure on 32bit

size_t may be a 32bit type in which case this code will fail to compile
due to a failed overload. Use u64 instead which ensures we cover
conversion of 32 and 64bit types.

jon-morra-zefr/vowpal_wabbit,marko-asplund/vowpal_wabbit,raosudha89/vowpal_wabbit,danmelamed/vowpal_wabbit,michaelbrundage/vowpal_wabbit,danmelamed/vowpal_wabbit,trufanov-nok/vowpal_wabbit,marko-asplund/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,tzukuoh/vowpal_wabbit,raosudha89/vowpal_wabbit,lhoang29/vowpal_wabbit,danmelamed/vowpal_wabbit,martinpopel/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,digitalreasoning/vowpal_wabbit,lhoang29/vowpal_wabbit,tzukuoh/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,danmelamed/vowpal_wabbit,marko-asplund/vowpal_wabbit,pombredanne/vowpal_wabbit,trufanov-nok/vowpal_wabbit,zachmayer/vowpal_wabbit,lhoang29/vowpal_wabbit,michaelbrundage/vowpal_wabbit,marko-asplund/vowpal_wabbit,deaktator/vowpal_wabbit,tzukuoh/vowpal_wabbit,pombredanne/vowpal_wabbit,pombredanne/vowpal_wabbit,lhoang29/vowpal_wabbit,digitalreasoning/vowpal_wabbit,trufanov-nok/vowpal_wabbit,martinpopel/vowpal_wabbit,digitalreasoning/vowpal_wabbit,michaelbrundage/vowpal_wabbit,lmatthieu/myvw,raosudha89/vowpal_wabbit,deaktator/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lhoang29/vowpal_wabbit,hal3/vowpal_wabbit,danmelamed/vowpal_wabbit,zachmayer/vowpal_wabbit,lmatthieu/myvw,hal3/vowpal_wabbit,pombredanne/vowpal_wabbit,lmatthieu/myvw,martinpopel/vowpal_wabbit,raosudha89/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,lhoang29/vowpal_wabbit,tzukuoh/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,hal3/vowpal_wabbit,zachmayer/vowpal_wabbit,deaktator/vowpal_wabbit,martinpopel/vowpal_wabbit,zachmayer/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,hal3/vowpal_wabbit,tzukuoh/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,raosudha89/vowpal_wabbit,hal3/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lmatthieu/myvw,zachmayer/vowpal_wabbit,trufanov-nok/vowpal_wabbit,trufanov-nok/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,raosudha89/vowpal_wabbit,danmelamed/vowpal_wabbit,zachmayer/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,trufanov-nok/vowpal_wabbit,danmelamed/vowpal_wabbit,hal3/vowpal_wabbit,zachmayer/vowpal_wabbit,trufanov-nok/vowpal_wabbit,michaelbrundage/vowpal_wabbit,martinpopel/vowpal_wabbit,marko-asplund/vowpal_wabbit,lhoang29/vowpal_wabbit,tzukuoh/vowpal_wabbit,deaktator/vowpal_wabbit,digitalreasoning/vowpal_wabbit,hal3/vowpal_wabbit,marko-asplund/vowpal_wabbit,danmelamed/vowpal_wabbit,pombredanne/vowpal_wabbit,marko-asplund/vowpal_wabbit,danmelamed/vowpal_wabbit,hal3/vowpal_wabbit,lmatthieu/myvw,digitalreasoning/vowpal_wabbit,martinpopel/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,michaelbrundage/vowpal_wabbit,hal3/vowpal_wabbit,digitalreasoning/vowpal_wabbit,zachmayer/vowpal_wabbit,deaktator/vowpal_wabbit,pombredanne/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,martinpopel/vowpal_wabbit,lmatthieu/myvw,raosudha89/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lhoang29/vowpal_wabbit,danmelamed/vowpal_wabbit,deaktator/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,tzukuoh/vowpal_wabbit,hal3/vowpal_wabbit,martinpopel/vowpal_wabbit,tzukuoh/vowpal_wabbit,digitalreasoning/vowpal_wabbit,digitalreasoning/vowpal_wabbit,deaktator/vowpal_wabbit,digitalreasoning/vowpal_wabbit,raosudha89/vowpal_wabbit,tzukuoh/vowpal_wabbit,michaelbrundage/vowpal_wabbit,pombredanne/vowpal_wabbit,zachmayer/vowpal_wabbit,lhoang29/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,michaelbrundage/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,raosudha89/vowpal_wabbit,trufanov-nok/vowpal_wabbit,zachmayer/vowpal_wabbit,deaktator/vowpal_wabbit,hal3/vowpal_wabbit,trufanov-nok/vowpal_wabbit,michaelbrundage/vowpal_wabbit,lhoang29/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,trufanov-nok/vowpal_wabbit,digitalreasoning/vowpal_wabbit,martinpopel/vowpal_wabbit,marko-asplund/vowpal_wabbit,pombredanne/vowpal_wabbit,raosudha89/vowpal_wabbit,lmatthieu/myvw,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,michaelbrundage/vowpal_wabbit,pombredanne/vowpal_wabbit,martinpopel/vowpal_wabbit,michaelbrundage/vowpal_wabbit,marko-asplund/vowpal_wabbit,tzukuoh/vowpal_wabbit,pombredanne/vowpal_wabbit,lmatthieu/myvw,marko-asplund/vowpal_wabbit