small speed enhancement in "fmelt". Replaced for-loops with 'memcpy' wherever possible.
small speed enhancement in "fmelt". Replaced for-loops with 'memcpy' wherever possible.

jangorecki/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table,Rdatatable/data.table,Rdatatable/data.table,vlulla/data.table,Rdatatable/data.table,Rdatatable/data.table,jangorecki/data.table,jangorecki/data.table