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.

tests/all/module.rs

@@ -2,6 +2,23 @@ use anyhow::Result;
 use std::io::BufWriter;
 use wasmtime::*;
 
+#[test]
+fn checks_incompatible_target() -> Result<()> {
+    let mut target = target_lexicon::Triple::host();
+    target.operating_system = target_lexicon::OperatingSystem::Unknown;
+    match Module::new(
+        &Engine::new(Config::new().target(&target.to_string())?)?,
+        "(module)",
+    ) {
+        Ok(_) => unreachable!(),
+        Err(e) => assert!(e
+            .to_string()
+            .contains("configuration does not match the host")),
+    }
+
+    Ok(())
+}
+
 #[test]
 fn caches_across_engines() {
     let c = Config::new();