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.

crates/environ/src/data_structures.rs

@@ -11,7 +11,7 @@ pub mod ir {
 
 pub mod settings {
     pub use cranelift_codegen::settings::{
-        builder, Builder, Configurable, Flags, OptLevel, SetError,
+        builder, Builder, Configurable, Flags, OptLevel, SetError, Setting, SettingKind,
     };
 }