Extensible diff types

[fpringle]

In this PR we let the user extend the built-in diff types with custom diffs via the SpecialDiff class.

This also subsumes the previous special list diff constructors DiffList and DiffNonEmpty.

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• [Refactor] move Doc and RenderOpts to Type module
• Export listDiffErrorDoc
• Remove renderListDiffError{With}
• Class for defining special cases for Diff, when a type doesnt quite fit the normal DiffError constructors
• Now we can implement SpecialDiff for lists and NonEmpty lists
• New modules for exporting Special stuff
• Update test suite
• Move NonEmpty instance to Special.List
• [Minor] export diffErrorNestedDoc
• specialDiffVia can be used to implement specialDiff
• [Refactor] Better organisation in Type.hs
• Replace specialDiffVia with gspecialDiffNested, which used functions from Generics.SOP.GGP
• [Unrelated] Rename showR -> showB
• Update docs