Fix example code in macros.md
Fix example code in macros.md

- Fix some function definitions by adding `(using QuoteContext)`
- Add a comment in the body of the `assertImpl` definition
- `showMeExpr` doesn't need a named QuoteContext, and use the anonymous form
- Adjust the order of the parameter lists of `evalExpr`

dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty