[Haskell-Servant] Upgrade to Servant 0.16

[f-f]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

Upgrade the Servant version from 0.15 to 0.16: this only shuffles some type names.

Note: this is a breaking change because people will be forced to upgrade to Servant 0.16 to use the new client

cc @jonschoning @wing328

[wing328]

Thanks for upgrading it to a newer version