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

docs/examples-rust-gcd.md

@@ -0,0 +1,22 @@
+# Calculating the GCD
+
+You can also [browse this source code online][code] and clone the wasmtime
+repository to run the example locally.
+
+[code]: https://github.com/bytecodealliance/wasmtime/blob/master/examples/gcd.rs
+
+This example shows off how run a wasm program which calculates the GCD of two
+numbers.
+
+## `gcd.wat`
+
+```wat
+{{#include ../examples/gcd.wat}}
+```
+
+
+## `gcd.rs`
+
+```rust,ignore
+{{#include ../examples/gcd.rs}}
+```