[postcard2]: Begin extracting heapless and embedded-io into dedicated side crates

[jamesmunns]

This is the first step towards "shattering" postcard to extract the "non-core" parts, like Flavor impls, into external crates that can be version bumped more often than the core crate.

I'm not sure if we really want N external crates, we might just want a postcard-extras crate that does more versioning, and can occassionally suffer a breaking change. If I'm already versioning all the deps, then putting these in modules instead of crates is probably not that huge of a deal.

We can make this decision after the full extraction is complete, that way we can see what the remain "postcard core" API looks like.

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[max-heller]

I'm not sure if we really want N external crates, we might just want a postcard-extras crate that does more versioning, and can occassionally suffer a breaking change. If I'm already versioning all the deps, then putting these in modules instead of crates is probably not that huge of a deal.

I haven't had a ton of time to think about it, but from an initial read, I'd think we want to either:

• have a single postcard-extras with feature-versioned deps
• have postcard-{eio,heapless,...} with unversioned deps (and version postcard-heapless alongside heapless itself)
  • this would be nice to avoid needing ::v0_9::to_vec() and the like

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[jamesmunns]

I appreciate the thoughts @max-heller! I'll come back to review this before releasing the accessory crates.