fwrite could as.data.table if given a matrix

[MichaelChirico]

Not sure how many people would prefer to have fwrite automatically know to apply as.data.table to a matrix (or, better, just accept a matrix and handle the "conversion" internally instead of doing so in R, which can be costly).

The former is easy to implement, the latter, a bit harder (since matrices are stored column-wise in memory, we'll have to do some index arithmetic to write row-wise)