Add support for `unums/posits`, a new floating point format

[vshymanskyy]

Posit is a number format that is similar to IEEE 754 format (floating point numbers).
The Posit Standard (2022) has been ratified by the Posit Working Group: https://posithub.org

The idea is to add a new posit tag that can apply to u8, u16 and u32 typed values and arrays, which are then treated as the corresponding Posits.
Also, this could be a good opportunity to add support for arbitrary-precision floats, but is there a rationale?