Support parsing the text format in wasmtime crate (#813)

* Support parsing the text format in `wasmtime` crate

This commit adds support to the `wasmtime::Module` type to parse the
text format. This is often quite convenient to support in testing or
tinkering with the runtime. Additionally the `wat` parser is pretty
lightweight and easy to add to builds, so it's relatively easy for us to
support as well!

The exact manner that this is now supported comes with a few updates to
the existing API:

* A new optional feature of the `wasmtime` crate, `wat`, has been added.
  This is enabled by default.
* The `Module::new` API now takes `impl AsRef<[u8]>` instead of just
  `&[u8]`, and when the `wat` feature is enabled it will attempt to
  interpret it either as a wasm binary or as the text format. Note that
  this check is quite cheap since you just check the first byte.
* A `Module::from_file` API was added as a convenience to parse a file
  from disk, allowing error messages for `*.wat` files on disk to be a
  bit nicer.
* APIs like `Module::new_unchecked` and `Module::validate` remain
  unchanged, they require the binary format to be called.

The intention here is to make this as convenient as possible for new
developers of the `wasmtime` crate. By changing the default behavior
though this has ramifications such as, for example, supporting the text
format implicitly through the C API now.

* Handle review comments

* Update more tests to avoid usage of `wat` crate

* Go back to unchecked for now in wasm_module_new

Looks like C# tests rely on this?

crates/api/examples/multi.rs

@@ -48,13 +48,9 @@ fn main() -> Result<()> {
     let engine = Engine::new(Config::new().wasm_multi_value(true));
     let store = Store::new(&engine);
 
-    // Load binary.
-    println!("Loading binary...");
-    let binary = wat::parse_str(WAT)?;
-
     // Compile.
     println!("Compiling module...");
-    let module = Module::new(&store, &binary).context("Error compiling module!")?;
+    let module = Module::new(&store, WAT).context("Error compiling module!")?;
 
     // Create external print functions.
     println!("Creating callback...");