Move all examples to a top-level directory (#1286)

* Move all examples to a top-level directory

This commit moves all API examples (Rust and C) to a top-level
`examples` directory. This is intended to make it more discoverable and
conventional as to where examples are located. Additionally all examples
are now available in both Rust and C to see how to execute the example
in the language you're familiar with. The intention is that as more
languages are supported we'd add more languages as examples here too.

Each example is also accompanied by either a `*.wat` file which is
parsed as input, or a Rust project in a `wasm` folder which is compiled
as input.

A simple driver crate was also added to `crates/misc` which executes all
the examples on CI, ensuring the C and Rust examples all execute
successfully.

examples/memory.rs

@@ -0,0 +1,97 @@
+//! An example of how to interact with wasm memory.
+//!
+//! Here a small wasm module is used to show how memory is initialized, how to
+//! read and write memory through the `Memory` object, and how wasm functions
+//! can trap when dealing with out-of-bounds addresses.
+
+// You can execute this example with `cargo run --example example`
+
+use anyhow::Result;
+use wasmtime::*;
+
+fn main() -> Result<()> {
+    // Create our `Store` context and then compile a module and create an
+    // instance from the compiled module all in one go.
+    let wasmtime_store = Store::default();
+    let module = Module::from_file(&wasmtime_store, "examples/memory.wat")?;
+    let instance = Instance::new(&module, &[])?;
+
+    // Load up our exports from the instance
+    let memory = instance
+        .get_export("memory")
+        .and_then(|e| e.memory())
+        .ok_or(anyhow::format_err!("failed to find `memory` export"))?;
+    let size = instance
+        .get_export("size")
+        .and_then(|e| e.func())
+        .ok_or(anyhow::format_err!("failed to find `size` export"))?
+        .get0::<i32>()?;
+    let load = instance
+        .get_export("load")
+        .and_then(|e| e.func())
+        .ok_or(anyhow::format_err!("failed to find `load` export"))?
+        .get1::<i32, i32>()?;
+    let store = instance
+        .get_export("store")
+        .and_then(|e| e.func())
+        .ok_or(anyhow::format_err!("failed to find `store` export"))?
+        .get2::<i32, i32, ()>()?;
+
+    // Note that these memory reads are *unsafe* due to unknown knowledge about
+    // aliasing with wasm memory. For more information about the safety
+    // guarantees here and how to use `Memory` safely, see the API
+    // documentation.
+    println!("Checking memory...");
+    assert_eq!(memory.size(), 2);
+    assert_eq!(memory.data_size(), 0x20000);
+    unsafe {
+        assert_eq!(memory.data_unchecked_mut()[0], 0);
+        assert_eq!(memory.data_unchecked_mut()[0x1000], 1);
+        assert_eq!(memory.data_unchecked_mut()[0x1003], 4);
+    }
+
+    assert_eq!(size()?, 2);
+    assert_eq!(load(0)?, 0);
+    assert_eq!(load(0x1000)?, 1);
+    assert_eq!(load(0x1003)?, 4);
+    assert_eq!(load(0x1ffff)?, 0);
+    assert!(load(0x20000).is_err()); // out of bounds trap
+
+    println!("Mutating memory...");
+    unsafe {
+        memory.data_unchecked_mut()[0x1003] = 5;
+    }
+
+    store(0x1002, 6)?;
+    assert!(store(0x20000, 0).is_err()); // out of bounds trap
+
+    unsafe {
+        assert_eq!(memory.data_unchecked_mut()[0x1002], 6);
+        assert_eq!(memory.data_unchecked_mut()[0x1003], 5);
+    }
+    assert_eq!(load(0x1002)?, 6);
+    assert_eq!(load(0x1003)?, 5);
+
+    // Grow memory.
+    println!("Growing memory...");
+    memory.grow(1)?;
+    assert_eq!(memory.size(), 3);
+    assert_eq!(memory.data_size(), 0x30000);
+
+    assert_eq!(load(0x20000)?, 0);
+    store(0x20000, 0)?;
+    assert!(load(0x30000).is_err());
+    assert!(store(0x30000, 0).is_err());
+
+    assert!(memory.grow(1).is_err());
+    assert!(memory.grow(0).is_ok());
+
+    println!("Creating stand-alone memory...");
+    let memorytype = MemoryType::new(Limits::new(5, Some(5)));
+    let memory2 = Memory::new(&wasmtime_store, memorytype);
+    assert_eq!(memory2.size(), 5);
+    assert!(memory2.grow(1).is_err());
+    assert!(memory2.grow(0).is_ok());
+
+    Ok(())
+}