Add a compile command to Wasmtime.

This commit adds a `compile` command to the Wasmtime CLI.

The command can be used to Ahead-Of-Time (AOT) compile WebAssembly modules.

With the `all-arch` feature enabled, AOT compilation can be performed for
non-native architectures (i.e. cross-compilation).

The `Module::compile` method has been added to perform AOT compilation.

A few of the CLI flags relating to "on by default" Wasm features have been
changed to be "--disable-XYZ" flags.

A simple example of using the `wasmtime compile` command:

```text
$ wasmtime compile input.wasm
$ wasmtime input.cwasm
```

src/bin/wasmtime.rs

@@ -6,7 +6,7 @@
 use anyhow::Result;
 use structopt::{clap::AppSettings, clap::ErrorKind, StructOpt};
 use wasmtime_cli::commands::{
-    ConfigCommand, RunCommand, WasmToObjCommand, WastCommand, WASM2OBJ_AFTER_HELP,
+    CompileCommand, ConfigCommand, RunCommand, WasmToObjCommand, WastCommand, WASM2OBJ_AFTER_HELP,
 };
 
 /// Wasmtime WebAssembly Runtime
@@ -38,6 +38,8 @@ enum WasmtimeApp {
     // !!! IMPORTANT: if subcommands are added or removed, update `parse_module` in `src/commands/run.rs`. !!!
     /// Controls Wasmtime configuration settings
     Config(ConfigCommand),
+    /// Compiles a WebAssembly module.
+    Compile(CompileCommand),
     /// Runs a WebAssembly module
     Run(RunCommand),
     /// Translates a WebAssembly module to native object file
@@ -49,9 +51,10 @@ enum WasmtimeApp {
 
 impl WasmtimeApp {
     /// Executes the command.
-    pub fn execute(&self) -> Result<()> {
+    pub fn execute(self) -> Result<()> {
         match self {
             Self::Config(c) => c.execute(),
+            Self::Compile(c) => c.execute(),
             Self::Run(c) => c.execute(),
             Self::WasmToObj(c) => c.execute(),
             Self::Wast(c) => c.execute(),