Enable jitdump profiling support by default (#1310)

* Enable jitdump profiling support by default

This the result of some of the investigation I was doing for #1017. I've
done a number of refactorings here which culminated in a number of
changes that all amount to what I think should result in jitdump support being
enabled by default:

* Pass in a list of finished functions instead of just a range to
  ensure that we're emitting jit dump data for a specific module rather
  than a whole `CodeMemory` which may have other modules.
* Define `ProfilingStrategy` in the `wasmtime` crate to have everything
  locally-defined
* Add support to the C API to enable profiling
* Documentation added for profiling with jitdump to the book
* Split out supported/unsupported files in `jitdump.rs` to avoid having
  lots of `#[cfg]`.
* Make dependencies optional that are only used for `jitdump`.
* Move initialization up-front to `JitDumpAgent::new()` instead of
  deferring it to the first module.
* Pass around `Arc<dyn ProfilingAgent>` instead of
  `Option<Arc<Mutex<Box<dyn ProfilingAgent>>>>`

The `jitdump` Cargo feature is now enabled by default which means that
our published binaries, C API artifacts, and crates will support
profiling at runtime by default. The support I don't think is fully
fleshed out and working but I think it's probably in a good enough spot
we can get users playing around with it!

crates/profiling/src/jitdump_disabled.rs

@@ -0,0 +1,33 @@
+use crate::ProfilingAgent;
+use anyhow::{bail, Result};
+use wasmtime_environ::entity::PrimaryMap;
+use wasmtime_environ::wasm::DefinedFuncIndex;
+use wasmtime_environ::Module;
+use wasmtime_runtime::VMFunctionBody;
+
+/// Interface for driving the creation of jitdump files
+#[derive(Debug)]
+pub struct JitDumpAgent {
+    _private: (),
+}
+
+impl JitDumpAgent {
+    /// Intialize a JitDumpAgent and write out the header
+    pub fn new() -> Result<Self> {
+        if cfg!(feature = "jitdump") {
+            bail!("jitdump is not supported on this platform");
+        } else {
+            bail!("jitdump support disabled at compile time");
+        }
+    }
+}
+
+impl ProfilingAgent for JitDumpAgent {
+    fn module_load(
+        &self,
+        _module: &Module,
+        _functions: &PrimaryMap<DefinedFuncIndex, *mut [VMFunctionBody]>,
+        _dbg_image: Option<&[u8]>,
+    ) {
+    }
+}