Revert unsafe `@pure` annotations

Last year, we made several performance improvements to FixedPointDecimals, and a lot of them used `@pure` to enforce constant folding.

However, since then (as discussed in last year's JuliaCon talk [If Runtime isn't Funtime](https://www.youtube.com/watch?v=JCFej--XER0)), I've come to understand that the `@pure` annotations I've added here are unsafe, because FixedPointDecimals supports _arbitrary_ integer types, and it's not safe to use `@pure` with _user-defined_ types.

We should revert these annotations before we can make another release.

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Long term:
- It _would_ be safe to use `@pure` if we restricted FixedPointDecimals to the built-in `Base.BitInteger` types, so we could consider separately defining a version of these functions for the built-in types that _are_ `@pure` and a non-pure version as the fallback.
- Or we could try to put more work into [StagedFunctions.jl](https://github.com/NHDaly/StagedFunctions.jl), which could be used to provide the same level of performance but currently adds non-trivial compilation time overhead.
- Or we could do some more thinking to see if there's another solution.

For now, we should probably just revert the annotations, at least.