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

fuzz/fuzz_targets/instantiate_translated.rs

@@ -1,12 +1,12 @@
 #![no_main]
 
 use libfuzzer_sys::fuzz_target;
+use wasmtime::Strategy;
 use wasmtime_fuzzing::{generators, oracles, with_log_wasm_test_case};
-use wasmtime_jit::CompilationStrategy;
 
 fuzz_target!(|data: generators::WasmOptTtf| {
     with_log_wasm_test_case!(&data.wasm, |wasm| oracles::instantiate(
         wasm,
-        CompilationStrategy::Auto
+        Strategy::Auto,
     ));
 });