Added TODOs from IdeSlave.hs
Lifted TODOs and FIXMEs from clause and instance elaborator
Removed old comment from `src/Idris/Elab/Data.hs`
Added TODOs and FIXMEs from the Elaborator for Data.
Lifted the TODOs from Primitives.hs
Merge pull request #1618 from jfdm/tidyup/lift-todos-3

More lifting of TODO/FIXMEs from the source.