Add map functions to iterator module

[tranzystorekk]

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Currently from_map* functions are there only for Erlang because I haven't figured out how to work with the gleam_stdlib.js one.

[lpil]

I'm not sure about this one. If we do not have a way to implement it for all platform we cannot merge it, and I'm not sure that we will be able to implement this for the JavaScript one, at least not until we have a less hacky implementation. https://github.com/gleam-lang/gleam/issues/1262

[tranzystorekk]

I can convert this to draft until then. I don't think the Erlang-side implementation is going to change much.

[lpil]

Closing due to inactivity. Please feel free to reopen