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/lib.rs

@@ -1,7 +1,37 @@
+//! The Wasmtime command line interface (CLI) crate.
+//!
+//! This crate implements the Wasmtime command line tools.
+
+#![deny(
+    missing_docs,
+    trivial_numeric_casts,
+    unused_extern_crates,
+    unstable_features
+)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../clippy.toml")))]
+#![cfg_attr(feature = "cargo-clippy", allow(clippy::new_without_default))]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    warn(
+        clippy::float_arithmetic,
+        clippy::mut_mut,
+        clippy::nonminimal_bool,
+        clippy::option_map_unwrap_or,
+        clippy::option_map_unwrap_or_else,
+        clippy::unicode_not_nfc,
+        clippy::use_self
+    )
+)]
+
+pub mod commands;
+
 use anyhow::{bail, Result};
+use std::path::PathBuf;
+use structopt::StructOpt;
 use wasmtime::Strategy;
 
-pub fn pick_compilation_strategy(cranelift: bool, lightbeam: bool) -> Result<Strategy> {
+fn pick_compilation_strategy(cranelift: bool, lightbeam: bool) -> Result<Strategy> {
     Ok(match (lightbeam, cranelift) {
         (true, false) => Strategy::Lightbeam,
         (false, true) => Strategy::Cranelift,
@@ -10,7 +40,7 @@ pub fn pick_compilation_strategy(cranelift: bool, lightbeam: bool) -> Result<Str
     })
 }
 
-pub fn init_file_per_thread_logger(prefix: &'static str) {
+fn init_file_per_thread_logger(prefix: &'static str) {
     file_per_thread_logger::initialize(prefix);
 
     // Extending behavior of default spawner:
@@ -35,3 +65,39 @@ pub fn init_file_per_thread_logger(prefix: &'static str) {
         .build_global()
         .unwrap();
 }
+
+/// Common options for commands that translate WebAssembly modules
+#[derive(StructOpt)]
+struct CommonOptions {
+    /// Use specified configuration file
+    #[structopt(long, parse(from_os_str), value_name = "CONFIG_PATH")]
+    config: Option<PathBuf>,
+
+    /// Use Cranelift for all compilation
+    #[structopt(long, conflicts_with = "lightbeam")]
+    cranelift: bool,
+
+    /// Enable debug output
+    #[structopt(short, long)]
+    debug: bool,
+
+    /// Generate debug information
+    #[structopt(short = "g")]
+    debug_info: bool,
+
+    /// Disable cache system
+    #[structopt(long)]
+    disable_cache: bool,
+
+    /// Enable support for proposed SIMD instructions
+    #[structopt(long)]
+    enable_simd: bool,
+
+    /// Use Lightbeam for all compilation
+    #[structopt(long, conflicts_with = "cranelift")]
+    lightbeam: bool,
+
+    /// Run optimization passes on translated functions
+    #[structopt(short = "O", long)]
+    optimize: bool,
+}