Constriction

Allow 32-bit floating point representations for probabilities in the python API. This considerably simplifies prototyping.

The Rust API still uses 64 bit floating point precision since (i) internal calculations in the `probability` crate are done on `f64`s anyway, and (ii) the Rust API mostly favors iterators over vectors, so conversion to `f64` does not require any copying.

Update rust dependency on `probability` to version 0.20, which now supports `no_std`. This is a breaking change since `constriction` exposes traits from `probability` through its public API.

We also now verify `no_std` compatibility in all CI workflows.

Fix an issue where constructing a categorical probability distribution sometimes lead to an infinite loop (see Issue 20).

Add support for recently released Python 3.11.

Add support for (outdated) Python 3.6 where possible (Linux + Mac).

Adds a .clone() method to all coder in the Python API, and implements Clone for RangeDecoder in the Rust API, which had been left out as an oversight.