Add support for type-level "strings".
These are types that look like "this" and "that". They are of kind `Symbol`, defined in module `GHC.TypeLits`. For each type-level symbol `X`, we have a singleton type, `TSymbol X`. The value of the singleton type can be named with the overloaded constant `tSymbol`. Here is an example: tSymbol :: TSymbol "Hello"
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