Add comparison operators on Decimal type in @cosmjs/math

[webmaster128]

There should be at least =, <, <=, >, >= on the Decimal type, which does not require unprecise conversion to JS numbers.

Since there is no operator overloading, I suggest interfaces like

a.equals(b)
a.isLessThan(b)
a.isGreaterThanOrEqual(b)

We need this here:

  /** true iff the distributor account needs a refill */
  public needsRefill(account: MinimalAccount, tickerSymbol: string): boolean {
    const meta = this.getTokenMeta(tickerSymbol);

    const balanceAmount = account.balance.find((b) => b.denom === meta.denom);

    const balance = balanceAmount
      ? Decimal.fromAtomics(balanceAmount.amount, meta.fractionalDigits)
      : Decimal.fromAtomics("0", 0);

    const thresholdAmount = this.refillThreshold(tickerSymbol);
    const threshold = Decimal.fromAtomics(thresholdAmount.amount, meta.fractionalDigits);

    // TODO: perform < operation on Decimal type directly
    // https://github.com/iov-one/iov-core/issues/1375
    return balance.toFloatApproximation() < threshold.toFloatApproximation();
  }

It would be good to have a compare funtion that we can use with sort like this

values.sort(Decimal.compare);

and use this to implement the other helpers

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Bonus: Extract the function signatures in and interface that we can reuse for the integer types.