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`).

src/commands/config.rs

@@ -0,0 +1,49 @@
+//! The module that implements the `wasmtime config` command.
+
+use anyhow::{anyhow, Result};
+use structopt::StructOpt;
+use wasmtime_environ::cache_create_new_config;
+
+const CONFIG_NEW_AFTER_HELP: &str =
+    "If no file path is specified, the system configuration file path will be used.";
+
+/// Controls Wasmtime configuration settings
+#[derive(StructOpt)]
+#[structopt(name = "run")]
+pub enum ConfigCommand {
+    /// Creates a new Wasmtime configuration file
+    #[structopt(after_help = CONFIG_NEW_AFTER_HELP)]
+    New(ConfigNewCommand),
+}
+
+impl ConfigCommand {
+    /// Executes the command.
+    pub fn execute(&self) -> Result<()> {
+        match self {
+            Self::New(c) => c.execute(),
+        }
+    }
+}
+
+/// Creates a new Wasmtime configuration file
+#[derive(StructOpt)]
+#[structopt(name = "new", after_help = CONFIG_NEW_AFTER_HELP)]
+pub struct ConfigNewCommand {
+    /// The path of the new configuration file
+    #[structopt(index = 1, value_name = "FILE_PATH")]
+    path: Option<String>,
+}
+
+impl ConfigNewCommand {
+    /// Executes the command.
+    pub fn execute(&self) -> Result<()> {
+        let path = cache_create_new_config(self.path.as_ref()).map_err(|e| anyhow!(e))?;
+
+        println!(
+            "Successfully created a new configuation file at '{}'.",
+            path.display()
+        );
+
+        Ok(())
+    }
+}