Add support for the experimental wasi-crypto APIs (#2597)

* Add support for the experimental wasi-crypto APIs

The sole purpose of the implementation is to allow bindings and
application developers to test the proposed APIs.

Rust and AssemblyScript bindings are also available as examples.

Like `wasi-nn`, it is currently disabled by default, and requires
the `wasi-crypto` feature flag to be compiled in.

* Rename the wasi-crypto/spec submodule

* Add a path dependency into the submodule for wasi-crypto

* Tell the publish script to vendor wasi-crypto

crates/wasi-crypto/README.md

@@ -0,0 +1,56 @@
+# wasmtime-wasi-crypto
+
+This crate enables support for the [wasi-crypto] APIs in Wasmtime.
+
+The sole purpose of the implementation is to allow bindings and
+application developers to test the proposed APIs. This implementation
+is not meant to be used in production. Like the specification, it is
+currently experimental and its functionality can quickly change.
+
+Since the [wasi-crypto] API is expected to be an optional feature of
+WASI, this crate is currently separate from the [wasi-common] crate.
+
+* [documentation]
+* [interfaces reference]
+* [interfaces reference (compact)]
+
+[wasi-crypto]: https://github.com/WebAssembly/wasi-crypto
+[wasi-common]: ../../wasi-common
+[documentation]: ../spec/docs/wasi-crypto.md
+[interfaces reference]: ../spec/witx/wasi_ephemeral_crypto.md
+[interfaces reference (compact)]: ../spec/witx/wasi_ephemeral_crypto.txt
+
+## Wasmtime integration
+
+Use the Wasmtime APIs to instantiate a Wasm module and link the
+`wasi-crypto` modules as follows:
+
+```rust
+use wasmtime_wasi_crypto::{
+    WasiCryptoAsymmetricCommon, WasiCryptoCommon, WasiCryptoCtx, WasiCryptoSignatures,
+    WasiCryptoSymmetric,
+};
+
+let cx_crypto = WasiCryptoCtx::new();
+WasiCryptoCommon::new(linker.store(), cx_crypto.clone()).add_to_linker(linker)?;
+WasiCryptoAsymmetricCommon::new(linker.store(), cx_crypto.clone()).add_to_linker(linker)?;
+WasiCryptoSignatures::new(linker.store(), cx_crypto.clone()).add_to_linker(linker)?;
+WasiCryptoSymmetric::new(linker.store(), cx_crypto.clone()).add_to_linker(linker)?;
+
+let wasi = wasmtime_wasi::old::snapshot_0::Wasi::new(linker.store(), mk_cx()?);
+wasi.add_to_linker(linker)?;
+```
+
+## Building Wasmtime
+
+Wasmtime must be compiled with the `wasi-crypto` feature flag
+(disabled by default) in order to include the crypto APIs.
+
+## Examples
+
+Example [rust bindings] and [assemblyscript bindings] are provided to
+demonstrate how these APIs can be used and exposed to applications in
+an idiomatic way.
+
+[rust bindings]: ../spec/implementations/bindings/rust
+[assemblyscript bindings]: ../spec/implementations/bindings/assemblyscript