Add Cursor type

This is a no_std variant of `std::io::Cursor`.

Fixes #631.

The implementation was generated by an LLM by copying the std impl and
adapting it. I (Zeeshan) reviewed the code and adjusted small bits. The
tests added are further guarantee that the code is doing what it's
supposed to do.

Author: zeenix

embedded-io-adapters/CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+- Add `Cursor` type, an `embedded-io` equivalent of `std::io::Cursor`.
+- Add `alloc` and `defmt` Cargo features.
+
 ## 0.7.0 - 2025-09-30
 
 - Update to embedded-io and embedded-io-async 0.7

embedded-io-adapters/Cargo.toml

@@ -13,17 +13,20 @@ categories = [
 ]
 
 [features]
-std = ["embedded-io/std"]
+std = ["alloc", "embedded-io/std"]
+alloc = ["embedded-io/alloc"]
+defmt = ["dep:defmt", "embedded-io/defmt"]
 tokio-1 = ["std", "dep:tokio", "dep:embedded-io-async", "embedded-io-async?/std"]
 futures-03 = ["std", "dep:futures", "dep:embedded-io-async", "embedded-io-async?/std"]
 
 [dependencies]
 embedded-io = { version = "0.7", path = "../embedded-io" }
 embedded-io-async = { version = "0.7", path = "../embedded-io-async", optional = true }
 
+defmt = { version = "1", optional = true }
 futures = { version = "0.3.21", features = ["std"], default-features = false, optional = true }
 tokio = { version = "1", features = ["io-util"], default-features = false, optional = true }
 
 [package.metadata.docs.rs]
-features = ["std", "tokio-1", "futures-03"]
+features = ["std", "defmt", "tokio-1", "futures-03"]
 rustdoc-args = ["--cfg", "docsrs"]