feat(composition): Adding an example that shows composition of isolated components

component-model/examples/composition/README.md

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+## Summary:
+This directory contains an example where component can be written and built separately in different languages and then composed together.
+There are multiple components in this repo :
+add, calculator and app are components written in RUST.
+sub is a component written in tinygo.
+
+calculator is a component that composes the components add and sub at compile time.
+calculator is a library component and cannot be used as a standalone app.
+Hence, the app component is built as a command component that composes the calculator component.
+
+## Prerequisites:
+
+### Install Wasmtime
+curl https://wasmtime.dev/install.sh -sSf | bash
+
+### Update PATH
+Wasmtime binary gets deployed at ~/.wasmtime directory.
+
+Update bash profile with wasmtime
+```export PATH="~/.wasmtime/bin:$PATH"```
+
+
+### Install cargo component 
+```cargo install cargo-component```
+
+
+## Build and test
+
+### Build the components 
+
+```
+cd calculator 
+cargo component build --release
+cd ..
+cd add 
+cargo component build --release
+cd ..
+
+cd app 
+cargo component build --release
+cd ..
+```
+
+### Subtractor component is a go component. Build it according to the readme in the sub directory to generate output component- sub-component.wasm.
+```
+cd sub
+```
+
+### Compose the components add, sub and calculator
+```
+cd ..
+wasm-tools compose calculator/target/wasm32-wasi/release/calculator.wasm -d add/target/wasm32-wasi/release/add.wasm -d sub/sub-component.wasm -o composed-add-sub.wasm
+```
+
+### Generate app component using composed-add-sub.wasm
+```
+wasm-tools compose app/target/wasm32-wasi/release/app.wasm -d composed-add-sub.wasm -o command.wasm
+```
+
+
+### Run the command component using wasmtime to test it 
+``` 
+wasmtime run command.wasm 1 2 add
+
+wasmtime run command.wasm 1 2 sub
+```

component-model/examples/composition/add/Cargo.toml

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+[package]
+name = "add"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+wit-bindgen-rt = { version = "0.24.0", features = ["bitflags"] }
+
+[lib]
+crate-type = ["cdylib"]
+
+[package.metadata.component]
+package = "component:add"
+
+[package.metadata.component.dependencies]

component-model/examples/composition/add/src/lib.rs

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+#[allow(warnings)]
+mod bindings;
+
+use bindings::exports::docs::adder::add::Guest;
+
+struct Component;
+
+impl Guest for Component {
+    fn add(x: u32, y: u32) -> u32 {
+           x + y
+       }
+}
+
+bindings::export!(Component with_types_in bindings);

component-model/examples/composition/add/wit/world.wit

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+package docs:adder;
+
+
+interface add {
+    add: func(a: u32, b: u32) -> u32;
+}
+
+world adder {
+    export add;
+}