Adds some test coverage to the FieldType parser that I noticed was not currently covered by tests currently. Specifically around the nesting of parens and square brackets. I was spiking a refactor of the parser in Quasi, and my implementation failed to parse this kind of thing while the current one does parse this successfully, so I thought it was worthwhile submitting this as increased coverage that can be merged into master to cover any future changes.

Before submitting your PR, check that you've:

• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock
• Ran stylish-haskell on any changed files.
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• Update the Changelog.md file with a link to your PR
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