SmartBigRational: a big rational type optimized for addition, subtraction and multiplication

This Rust library provides the SmartBigRational type, similar to the num crate's BigRational type, but optimized for addition, subtraction and multiplication operations.

Under the hood, the denominator is represented as a product of small primes, which allows fast Greatest Common Divisor (GCD) operations, which are often the bottleneck with the vanilla BigRational type. Additionally, SmartBigRational aren't always represented as reduced fractions, which again avoids GCD operations.

The underlying technique is described in the blog post Optimization adventures: making a parallel Rust workload even faster with data-oriented design (and other tricks).

On the flip side, memory usage with non-reduced SmartBigRational fractions can increase, notably when computing an expression with high multiplicative depth. In these cases, vanilla BigRational may be more efficient.

In practice, SmartBigRational are well suited for expressions with long sums, low multiplicative depth and/or small denominators.

As always: benchmark before choosing one or the other option.