Remove usage of CompilationStrategy from Config (#764)

* Remove usage of `CompilationStrategy` from `Config`

This commit removes the public API usage of the internal
`CompilationStrategy` enumeration from the `Config` type in the
`wasmtime` crate. To do this the `enum` was copied locally into the
crate and renamed `Strategy`. The high-level description of this change
is:

* The `Config::strategy` method now takes a locally-defined `Strategy`
  enumeration instead of an internal type.

* The contents of `Strategy` are always the same, not relying on Cargo
  features to indicate which variants are present. This avoids
  unnecessary downstream `#[cfg]`.

* A `lightbeam` feature was added to the `wasmtime` crate itself to
  lightbeam compilation support.

* The `Config::strategy` method is now fallible. It returns a runtime
  error if support for the selected strategy wasn't compiled in.

* The `Strategy` enum is listed as `#[non_exhaustive]` so we can safely
  add variants over time to it.

This reduces the public crate dependencies of the `wasmtime` crate
itself, removing the need to reach into internal crates even more!

cc #708

* Fix fuzz targets

* Update nightly used to build releases

* Run rustfmt

crates/obj/src/data_segment.rs

@@ -1,3 +1,4 @@
+use anyhow::Result;
 use faerie::{Artifact, Decl};
 use wasmtime_environ::DataInitializer;
 
@@ -6,10 +7,9 @@ pub fn declare_data_segment(
     obj: &mut Artifact,
     _data_initaliazer: &DataInitializer,
     index: usize,
-) -> Result<(), String> {
+) -> Result<()> {
     let name = format!("_memory_{}", index);
-    obj.declare(name, Decl::data())
-        .map_err(|err| format!("{}", err))?;
+    obj.declare(name, Decl::data())?;
     Ok(())
 }
 
@@ -18,9 +18,8 @@ pub fn emit_data_segment(
     obj: &mut Artifact,
     data_initaliazer: &DataInitializer,
     index: usize,
-) -> Result<(), String> {
+) -> Result<()> {
     let name = format!("_memory_{}", index);
-    obj.define(name, Vec::from(data_initaliazer.data))
-        .map_err(|err| format!("{}", err))?;
+    obj.define(name, Vec::from(data_initaliazer.data))?;
     Ok(())
 }