-- add driver.Value type and documentation,
   convert from interface{} to Value where
   appropriate.
-- don't say "subset" anywhere,
-- SubsetValuer -> Valuer
-- SubsetValue -> Value
-- IsParameterSubsetType -> IsValue
-- IsScanSubsetType -> IsScanValue

Fixes #2842

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5674084