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

Cargo.lock

@@ -2016,6 +2016,7 @@ dependencies = [
 name = "wasmtime-environ"
 version = "0.7.0"
 dependencies = [
+ "anyhow",
  "base64 0.11.0",
  "bincode",
  "cranelift-codegen",
@@ -2054,6 +2055,7 @@ dependencies = [
  "env_logger 0.7.1",
  "libfuzzer-sys",
  "log",
+ "wasmtime",
  "wasmtime-fuzzing",
  "wasmtime-jit",
 ]
@@ -2115,6 +2117,7 @@ dependencies = [
 name = "wasmtime-obj"
 version = "0.7.0"
 dependencies = [
+ "anyhow",
  "faerie",
  "more-asserts",
  "wasmtime-environ",