Revert unsafe `@pure` annotations

[NHDaly]

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), 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, 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.