attach locations to AST with attachSourcePosSource

We currently use [`getSourcePos`](https://hackage.haskell.org/package/megaparsec-8.0.0/docs/Text-Megaparsec.html#v:getSourcePos) to obtain current source location during parsing and annotate AST with it, but as the documentation says, `getSourcePos` is slow to use frequently. Using [`getOffset`](https://hackage.haskell.org/package/megaparsec-8.0.0/docs/Text-Megaparsec.html#v:getOffset) to annotate with offsets first and convert them to `SourcePos` later with [`attachSourcePosSource`](https://hackage.haskell.org/package/megaparsec-8.0.0/docs/Text-Megaparsec-Error.html#v:attachSourcePos) would be faster.