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/commands.rs

@@ -3,7 +3,8 @@
 mod compile;
 mod config;
 mod run;
+mod settings;
 mod wasm2obj;
 mod wast;
 
-pub use self::{compile::*, config::*, run::*, wasm2obj::*, wast::*};
+pub use self::{compile::*, config::*, run::*, settings::*, wasm2obj::*, wast::*};