Add a wasmtime settings command to print Cranelift settings.

This commit adds the `wasmtime settings` command to print out available
Cranelift settings for a target (defaults to the host).

The compile command has been updated to remove the Cranelift ISA options in
favor of encouraging users to use `wasmtime settings` to discover what settings
are available.  This will reduce the maintenance cost for syncing the compile
command with Cranelift ISA flags.

src/bin/wasmtime.rs

@@ -6,7 +6,7 @@
 use anyhow::Result;
 use structopt::{clap::AppSettings, clap::ErrorKind, StructOpt};
 use wasmtime_cli::commands::{
-    CompileCommand, ConfigCommand, RunCommand, WasmToObjCommand, WastCommand,
+    CompileCommand, ConfigCommand, RunCommand, SettingsCommand, WasmToObjCommand, WastCommand,
 };
 
 /// Wasmtime WebAssembly Runtime
@@ -42,6 +42,8 @@ enum WasmtimeApp {
     Compile(CompileCommand),
     /// Runs a WebAssembly module
     Run(RunCommand),
+    /// Displays available Cranelift settings for a target.
+    Settings(SettingsCommand),
     /// Translates a WebAssembly module to native object file
     #[structopt(name = "wasm2obj")]
     WasmToObj(WasmToObjCommand),
@@ -56,6 +58,7 @@ impl WasmtimeApp {
             Self::Config(c) => c.execute(),
             Self::Compile(c) => c.execute(),
             Self::Run(c) => c.execute(),
+            Self::Settings(c) => c.execute(),
             Self::WasmToObj(c) => c.execute(),
             Self::Wast(c) => c.execute(),
         }