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

src/bin/wasmtime.rs

@@ -257,11 +257,11 @@ fn main() -> Result<()> {
     }
 
     // Decide how to compile.
-    let strategy = pick_compilation_strategy(args.flag_cranelift, args.flag_lightbeam);
-
-    config
-        .flags(settings::Flags::new(flag_builder))
-        .strategy(strategy);
+    config.strategy(pick_compilation_strategy(
+        args.flag_cranelift,
+        args.flag_lightbeam,
+    )?)?;
+    config.flags(settings::Flags::new(flag_builder));
     let engine = Engine::new(&config);
     let store = HostRef::new(Store::new(&engine));