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/Cargo.toml

@@ -21,6 +21,7 @@ cfg-if = "0.1.9"
 backtrace = "0.3.42"
 rustc-demangle = "0.1.16"
 lazy_static = "1.4"
+wat = { version = "1.0.7", optional = true }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"
@@ -37,6 +38,8 @@ wat = "1.0"
 maintenance = { status = "actively-developed" }
 
 [features]
+default = ['wat']
+
 # Enables experimental support for the lightbeam codegen backend, an alternative
 # to cranelift. Requires Nightly Rust currently, and this is not enabled by
 # default.