reshape2:::melt, we melt data.frames and
matrixs. This function is built for speed.
melt_(data, ...)
"melt_"(data, id.vars, measure.vars, variable.name = "variable", ..., value.name = "value")
"melt_"(data, ...)data.frame to melt.measure.vars.id.vars.logical > integer > numeric >
character.