Fixes #5379 - dcast with factor cols on LHS along with drop=FALSE

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1172 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
+ Faster := and set() to a new column by not filling with NA first where we know entire vector will be written
+ set() no longer duplicates atomic NAMED==0 plonked columns

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1173 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Trapped DT[,sum(.SD),by=] with error suggesting lapply(.SD,sum) or base::sum(.SD), #5380.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1174 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'

Conflicts:
	pkg/inst/tests/tests.Rraw
Removed bug report on unreleased features