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?

tests/custom_signal_handler.rs

@@ -107,8 +107,7 @@ mod tests {
     fn test_custom_signal_handler_single_instance() -> Result<()> {
         let engine = Engine::new(&Config::default());
         let store = Store::new(&engine);
-        let data = wat::parse_str(WAT1)?;
-        let module = Module::new(&store, &data)?;
+        let module = Module::new(&store, WAT1)?;
         let instance = Instance::new(&module, &[])?;
 
         let (base, length) = set_up_memory(&instance);
@@ -166,8 +165,7 @@ mod tests {
     fn test_custom_signal_handler_multiple_instances() -> Result<()> {
         let engine = Engine::new(&Config::default());
         let store = Store::new(&engine);
-        let data = wat::parse_str(WAT1)?;
-        let module = Module::new(&store, &data)?;
+        let module = Module::new(&store, WAT1)?;
 
         // Set up multiple instances
 
@@ -261,8 +259,7 @@ mod tests {
         let store = Store::new(&engine);
 
         // instance1 which defines 'read'
-        let data1 = wat::parse_str(WAT1)?;
-        let module1 = Module::new(&store, &data1)?;
+        let module1 = Module::new(&store, WAT1)?;
         let instance1 = Instance::new(&module1, &[])?;
         let (base1, length1) = set_up_memory(&instance1);
         unsafe {
@@ -277,8 +274,7 @@ mod tests {
         let instance1_read = instance1_exports[0].clone();
 
         // instance2 wich calls 'instance1.read'
-        let data2 = wat::parse_str(WAT2)?;
-        let module2 = Module::new(&store, &data2)?;
+        let module2 = Module::new(&store, WAT2)?;
         let instance2 = Instance::new(&module2, &[instance1_read])?;
         // since 'instance2.run' calls 'instance1.read' we need to set up the signal handler to handle
         // SIGSEGV originating from within the memory of instance1