Add a function to show a rational number as a decimal, to a given precision.

[Taneb]

I'd like a function to turn a rational into its decimal expansion at a given precision. Perhaps with type showDecimalAtPrecision : (precision : ℕ) → ℚ → String, although maybe it should also take an argument specifying the base.

[barrettj12]

I'm interested in solving this issue.

[MatthewDaggitt]

We already have some similar-ish code for converting to different bases in Data.DIgit if you're looking for inspiration.