chore(cargo/sdk): prep for publishing spin-sdk crate

[vdice]

Some updates in preparation for publishing the spin-sdk crate per https://doc.rust-lang.org/cargo/reference/publishing.html#before-publishing-a-new-crate

TODO:

• publish the spin-macro crate and update the reference in spin-sdk's Cargo.toml (chore(cargo/sdk): prep for publishing spin-sdk crate #2151 (comment))

Ref #316

[vdice]

Additions in advance of publishing the spin-macro crate in 2271d8a

Please feel free to suggest updates to the currently-minimal description and README for this crate.

Also, do we wish to keep this crate at a separate version cadence (currently 0.1.0) or do we want to align with Spin's (workspace) version?

[vdice]

(Pre-emptively bumped the spin-macro dep in spin-sdk to the to-be-published 0.1.0 version in d17ba1f; will re-run CI once published.)

[vdice]

The spin-macro crate is now published.

Alas, cargo publish --dry-run for the spin-sdk crate now results in errors re: the use of the relative path to the top-level wit directory:

error: failed to read file "/Users/vdice/go/src/github.com/fermyon/spin/target/package/spin-sdk-2.1.0-pre0/../../wit"

       Caused by:
           No such file or directory (os error 2)
  --> src/lib.rs:24:5
   |
24 | /     wit_bindgen::generate!({
25 | |         world: "platform",
26 | |         path: "../../wit",
27 | |     });
   | |______^
   |
   = note: this error originates in the macro `wit_bindgen::generate` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: could not find `wasi` in `wit`
 --> src/http/conversions.rs:5:17
  |
5 | use crate::wit::wasi::io::streams;
  |                 ^^^^ could not find `wasi` in `wit`

error[E0433]: failed to resolve: could not find `wasi` in `wit`
  --> src/http.rs:15:17
   |
15 | use super::wit::wasi::http::types;
   |                 ^^^^ could not find `wasi` in `wit`

error[E0433]: failed to resolve: could not find `wasi` in `wit`
  --> src/http.rs:16:17
   |
16 | use crate::wit::wasi::io::streams;
   |                 ^^^^ could not find `wasi` in `wit`
   ...

[vdice]

f48ebe2 adds a symlink'd wit dir in sdk/rust to address #2151 (comment)