Use structopt instead of docopt.

This commit refactors the Wasmtime CLI tools to use `structopt` instead of
`docopt`.

The `wasmtime` tool now has the following subcommands:

* `config new` - creates a new Wasmtime configuration file.
* `run` - runs a WebAssembly module.
* `wasm2obj` - translates a Wasm module to native object file.
* `wast` - runs a test script file.

If no subcommand is specified, the `run` subcommand is used. Thus,
`wasmtime foo.wasm` should continue to function as expected.

The `wasm2obj` and `wast` tools still exist, but delegate to the same
implementation as the `wasmtime` subcommands.  The standalone `wasm2obj` and
`wast` tools may be removed in the future in favor of simply using `wasmtime`.

Included in this commit is a breaking change to the default Wasmtime
configuration file: it has been renamed from `wasmtime-cache-config.toml` to
simply `config.toml`.  The new name is less specific which will allow for
additional (non-cache-related) settings in the future.

There are some breaking changes to improve command line UX:

* The `--cache-config` option has been renamed to `--config`.
* The `--create-config-file` option has moved to the `config new` subcommand.
As a result, the `wasm2obj` and `wast` tools cannot be used to create a new
config file.
* The short form of the `--optimize` option has changed from
`-o` to `-O` for consistency.
* The `wasm2obj` command takes the output object file as a
required positional argument rather than the former required output *option*
(e.g. `wasmtime wasm2obj foo.wasm foo.obj`).

crates/environ/src/cache/config.rs

@@ -151,7 +151,7 @@ pub fn create_new_config<P: AsRef<Path> + Debug>(config_file: Option<P>) -> Resu
 
     if config_file.exists() {
         bail!(
-            "Specified config file already exists! Path: {}",
+            "Configuration file '{}' already exists.",
             config_file.display()
         );
     }
@@ -193,7 +193,7 @@ lazy_static! {
         ProjectDirs::from("", "BytecodeAlliance", "wasmtime");
     static ref DEFAULT_CONFIG_PATH: Result<PathBuf, String> = PROJECT_DIRS
         .as_ref()
-        .map(|proj_dirs| proj_dirs.config_dir().join("wasmtime-cache-config.toml"))
+        .map(|proj_dirs| proj_dirs.config_dir().join("config.toml"))
         .ok_or_else(|| "Config file not specified and failed to get the default".to_string());
 }