Utility of Helper functions for String <==> Number conversions in the SDK?

[wyhaines]

This question came up in the developer-chat channel on the discord last week:

is there a way to convert a string to U256?
U256::from_str("123")
Stella tells me that there's a from_str_radix method, but i dont see it
only thing that could be useful is U256::from_be_bytes, but i would need to first convert my number to bytes. Is there a way to do that?
String.to_bytes doesn't work, since it's converting char by char
also, the bytes!() macro doesn't accept anything more than 2^128

I had built a small, specific set of conversion utilities that I needed for a soroban project/experiment that I've been working on for a couple of weeks, and so I fleshed out the pattern to cover the range of conversions, so you can write code like this:

  #[contractimpl]
  impl MyContract {
      pub fn set_amount(env: Env, amount: String) {
          // From &str literal
          let default = str_to_u256(&env, "1000").unwrap();

          // From soroban_sdk::String (e.g., form input)
          let user_amount = string_to_u256(&env, &amount).unwrap_or(default);

          // Do stuff with default and user_amount...
      }
  }

I have these functions in a crate that I've written, and they can be used via this crate (https://crates.io/crates/soroban-render-sdk) now, although they are definitely under-tested and I make no strong claims about the quality of the current implementation.

This brought up some short conversation about whether it would be useful to have functions like these directly in the rust version of the soroban-sdk, and @tupui as well as @leighmcculloch suggested that I open a discussion here to talk about it.

I am weakly opinionated about this. I think that when building applications that interact with contracts, conversions can be frequent, and so there should be some easily found set of well vetted conversion functions available for the developer to use. Having this directly in the SDK seems like the simplest thing, but I feel like they could be in a separate crate, whether that's like where my versions of these are now, in a crate with additional purposes, or it is a narrowly scoped, purpose built crate, though, I don't have any strong opinion about currently. I can see a push and a pull in both directions.