Avoid allocations in fmt machinery

[shamatar]

But still keep it under (different) feature flag

Motivation

Use of formatting in no_std environments (or environments where Global allocator is not desired)

Solution

Use stack-allocated buffer instead, with clear upper bound on number of possible limbs used for formatting

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[lightsing]

why this need to be gated behind a feature fmt?

[shamatar]

Previously it was locked under alloc, so it’s somewhat arbitrary

[prestwich]

Previously it was locked under alloc, so it’s somewhat arbitrary

The purpose of the alloc gate was to preserve maximum flexibility for no-std users with an alloc-free approach we can remove the feature-gating entirely

[shamatar]

Sure, I removed the feature gate

[prestwich]

related work that achieves the same goal has been included in #488