Optimize generated code via the CLI by default (#973)

* Optimize generated code via the CLI by default

This commit updates the behavior of the CLI and adds a new flag. It
first enables the `--optimize` flag by default, ensuring that usage of
the `wasmtime` CLI will enable cranelift optimizations by default. Next
it also adds a `--opt-level` flag which is similar to Rust's
`-Copt-level` where it takes a string argument of how to optimize. This
is updates to support 0/1/2/s, where 1 is currently the same as 2 but
added for consistency with other compilers. The default setting is
`--opt-level=2`.

When the `-O` flag is not passed the `--opt-level` flag is used,
otherwise `-O` takes precedent in the sense that it implies
`--opt-level=2` which is the highest optimization level. The thinking is
that these flags will in general select the highest optimization level
specified as the final optimization level.

* Add inline docs

* fix a test

src/obj.rs

@@ -19,7 +19,7 @@ pub fn compile_to_obj(
     target: Option<&Triple>,
     strategy: Strategy,
     enable_simd: bool,
-    optimize: bool,
+    opt_level: wasmtime::OptLevel,
     debug_info: bool,
     artifact_name: String,
     cache_config: &CacheConfig,
@@ -38,8 +38,15 @@ pub fn compile_to_obj(
         flag_builder.enable("enable_simd").unwrap();
     }
 
-    if optimize {
-        flag_builder.set("opt_level", "speed").unwrap();
+    match opt_level {
+        wasmtime::OptLevel::None => {}
+        wasmtime::OptLevel::Speed => {
+            flag_builder.set("opt_level", "speed").unwrap();
+        }
+        wasmtime::OptLevel::SpeedAndSize => {
+            flag_builder.set("opt_level", "speed_and_size").unwrap();
+        }
+        other => bail!("unknown optimization level {:?}", other),
     }
 
     let isa = isa_builder.finish(settings::Flags::new(flag_builder));