Changes how examples are tested, and fixes ExitError bug (#468)

Before, we tested the examples/ directory using "ExampleXX", but this is
not ideal because it literally embeds the call to `main` into example
godoc output. This stops doing that for a different infrastructure.

This also makes sure there's a godoc example for both the main package
and wasi, so that people looking at https://pkg.go.dev see something and
also a link to our real examples directory.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

example_test.go

@@ -0,0 +1,43 @@
+package wazero
+
+import (
+	_ "embed"
+	"fmt"
+	"log"
+)
+
+// This is an example of how to use WebAssembly via adding two numbers.
+//
+// See https://github.com/tetratelabs/wazero/tree/main/examples for more examples.
+func Example() {
+	// Create a new WebAssembly Runtime.
+	r := NewRuntime()
+
+	// Add a module to the runtime named "wasm/math" which exports one function "add", implemented in WebAssembly.
+	mod, err := r.InstantiateModuleFromCode([]byte(`(module $wasm/math
+    (func $add (param i32 i32) (result i32)
+        local.get 0
+        local.get 1
+        i32.add
+    )
+    (export "add" (func $add))
+)`))
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer mod.Close()
+
+	// Get a function that can be reused until its module is closed:
+	add := mod.ExportedFunction("add")
+
+	x, y := uint64(1), uint64(2)
+	results, err := add.Call(nil, x, y)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%s: %d + %d = %d\n", mod.Name(), x, y, results[0])
+
+	// Output:
+	// wasm/math: 1 + 2 = 3
+}