Add examples of linking and WASI (#1369)

* Add examples of linking and WASI

This commit adds two example programs, one for linking two modules
together and one for instantiating WASI. The linkage example
additionally uses WASI to get some meaningful output at this time.

cc #1272

* Add examples to the book as well

* More links!

* Ignore examples from rustdoc testsing

* More example updates

* More ignored

examples/linking1.wat

@@ -0,0 +1,23 @@
+(module
+  (import "linking2" "double" (func $double (param i32) (result i32)))
+  (import "linking2" "log" (func $log (param i32 i32)))
+  (import "linking2" "memory" (memory 1))
+  (import "linking2" "memory_offset" (global $offset i32))
+
+  (func (export "run")
+    ;; Call into the other module to double our number, and we could print it
+    ;; here but for now we just drop it
+    i32.const 2
+    call $double
+    drop
+
+    ;; Our `data` segment initialized our imported memory, so let's print the
+    ;; string there now.
+    global.get $offset
+    i32.const 14
+    call $log
+  )
+
+  (data (global.get $offset) "Hello, world!\n")
+)
+