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/src/callable.rs

@@ -30,7 +30,7 @@ use wasmtime_runtime::Export;
 /// # fn main () -> Result<(), Box<dyn std::error::Error>> {
 /// // Simple module that imports our host function ("times_two") and re-exports
 /// // it as "run".
-/// let binary = wat::parse_str(r#"
+/// let wat = r#"
 ///    (module
 ///      (func $times_two (import "" "times_two") (param i32) (result i32))
 ///      (func
@@ -40,11 +40,11 @@ use wasmtime_runtime::Export;
 ///        (local.get 0)
 ///        (call $times_two))
 ///    )
-/// "#)?;
+/// "#;
 ///
 /// // Initialise environment and our module.
 /// let store = wasmtime::Store::default();
-/// let module = wasmtime::Module::new(&store, &binary)?;
+/// let module = wasmtime::Module::new(&store, wat)?;
 ///
 /// // Define the type of the function we're going to call.
 /// let times_two_type = wasmtime::FuncType::new(