Move malloc(), free(), etc. from esp-wifi to esp-alloc

[AnthonyGrondin]

Motivations

esp-wifi, esp-alloc and esp-mbedtls are tightly coupled by their memory allocator implementation. esp-wifi needs an implementation of malloc() and free(), and since #2099, it uses Rust's Global allocator. esp-alloc is a provider of a global allocator for ESP SoCs but users are free to provide their own allocator if they want.

esp-mbedtls and esp-mbedtls-sys depend on esp-wifi, strictly for providing malloc() and free().

Solution

These should be moved to esp-alloc, to only depend on an allocation crate, if one only needs malloc(), free(), etc. They should be defined with conditional compilation and / or weak linkage to avoid conflict issues if another crate provides these symbols.

Alternatives

Provide a c-lib / c-compat / esp-c-compat crate to define and implement C symbols, with conditional compilation. It would not only provide memory allocation symbols, but any symbols such as vsnprintf().

Additional context

• esp-wifi: use even more ROM functions #2445
• Move binary logging to sys crate #2183
• https://github.com/esp-rs/esp-mbedtls/blob/1431835a9a3464820e97203347ec33e313f8e116/esp-mbedtls-sys/Cargo.toml#L18-L22

[Dominaezzz]

Some extra motive. esp-wifi won't be available on the P4 (No WiFi or Bluetooth) but there's no reason esp-mbedtls shouldn't work there. The hard dependency on esp-wifi gets in the way.

[AnthonyGrondin]

Ideally esp-mbedtls shouldn't depend on any ESP crates, because in the end, it's just a transport layer, over embedded-io(-async) Read and Write. We will provide hardware acceleration and such using crypto providers, the same way rustls and embedded-tls does. With a few refactors, it will be completely platform agnostic, and can work on any transport medium.