sugar functions no longer mask by default; see extended

Rather than masking the base R functions (which is occasionally a bad
idea because eg. RcppSugar::unique does not function the same as
base::unique; ie, order is different), functions are now all exported
by default with the prefix "sugar_", and two utility functions
attach_sugar() and detach_sugar() can be used to attach, detach the
functions (thus masking the base R functions again)