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/module.rs

@@ -2,6 +2,7 @@ use crate::context::layout_vmcontext;
 use crate::data_segment::{declare_data_segment, emit_data_segment};
 use crate::function::{declare_functions, emit_functions};
 use crate::table::{declare_table, emit_table};
+use anyhow::Result;
 use faerie::{Artifact, Decl, Link};
 use wasmtime_environ::isa::TargetFrontendConfig;
 use wasmtime_environ::{Compilation, DataInitializer, Module, Relocations};
@@ -10,18 +11,16 @@ fn emit_vmcontext_init(
     obj: &mut Artifact,
     module: &Module,
     target_config: &TargetFrontendConfig,
-) -> Result<(), String> {
+) -> Result<()> {
     let (data, table_relocs) = layout_vmcontext(module, target_config);
-    obj.declare_with("_vmcontext_init", Decl::data().global(), data.to_vec())
-        .map_err(|err| format!("{}", err))?;
+    obj.declare_with("_vmcontext_init", Decl::data().global(), data.to_vec())?;
     for reloc in table_relocs.iter() {
         let target_name = format!("_table_{}", reloc.index);
         obj.link(Link {
             from: "_vmcontext_init",
             to: &target_name,
             at: reloc.offset as u64,
-        })
-        .map_err(|err| format!("{}", err))?;
+        })?;
     }
     Ok(())
 }
@@ -35,7 +34,7 @@ pub fn emit_module(
     relocations: &Relocations,
     data_initializers: &[DataInitializer],
     target_config: &TargetFrontendConfig,
-) -> Result<(), String> {
+) -> Result<()> {
     declare_functions(obj, module, relocations)?;
 
     for (i, initializer) in data_initializers.iter().enumerate() {