Add dec!() macro by example

[daniel-pfeiffer]

As discussed, here is a cleaned up, further optimised and integrated new implementation of dec!. It is a completely const and compile-time drop in replacement. This requires Rust 1.79.

Your Result<…String…> is not destructible, i.e. not usable in const. Therefore the new parser has its own Result, that can be transparently converted by ?. This also required splitting try_from_i128_with_scale which thus became simpler.

For now the parser does only exact in i128. Rounding should be addable, if you want to switch parsing completely to this.

Being generic on a smaller type doesn’t currently seem possible in const. Should a benchmark show this to be slower than parsing into u64, the old and new parsers might continue to coexist, till a solution is found.