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

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1183 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Reverting memcpy on STRSXPs to for-loops. Thanks Matt.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1184 edb9625f-4e0d-4859-8d74-9fd3b1da38cb