Avoid using memcpy on STRSXPs
As per correspondence with Matt Dowle, SET_STRING_ELT ensures that objects are properly aged (see CHECK_OLD_TO_NEW in SET_STRING_ELT, in memory.c). If we just use memcpy, because CHARSXPs are SEXPs, there is a chance they will be gc'ed if R needs to shrink the global string cache, and a raw memcpy won't properly protect them from the gc.
Identify two minor changes in reshape2
Add a few helper links in @seealso sections
Add tests that melt preserves dimnames of arrays and tables