Add new maintainer & package MapWith

[davjam]

Checklist:

• Meaningful commit message, eg add my-cool-package (please not mention build-constraints.yml)

• At least 30 minutes have passed since uploading to Hackage

• On your own machine, in a new directory, you have successfully run the following set of commands (replace $package with the name of the package that is submitted, and $version with the version of the package you want to get into Stackage):

  stack unpack $package-$version  # $version is optional
  cd $package-$version
  rm -f stack.yaml && stack init --resolver nightly
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks
  

(I got several "Warning: Failed to decode module interface:" warnings, but think that's expected right now given commercialhaskell/stack#5134)