Add initial support for the multi-memory proposal (#2263)

This commit adds initial (gated) support for the multi-memory wasm
proposal. This was actually quite easy since almost all of wasmtime
already expected multi-memory to be implemented one day. The only real
substantive change is the `memory.copy` intrinsic changes, which now
accounts for the source/destination memories possibly being different.

src/lib.rs

@@ -124,6 +124,10 @@ struct CommonOptions {
     #[structopt(long)]
     enable_bulk_memory: Option<bool>,
 
+    /// Enable support for the multi-memory proposal
+    #[structopt(long)]
+    enable_multi_memory: bool,
+
     /// Enable all experimental Wasm features
     #[structopt(long)]
     enable_all: bool,
@@ -194,6 +198,7 @@ impl CommonOptions {
             )
             .wasm_multi_value(self.enable_multi_value.unwrap_or(true) || self.enable_all)
             .wasm_threads(self.enable_threads || self.enable_all)
+            .wasm_multi_memory(self.enable_multi_memory || self.enable_all)
             .cranelift_opt_level(self.opt_level())
             .strategy(pick_compilation_strategy(self.cranelift, self.lightbeam)?)?
             .profiler(pick_profiling_strategy(self.jitdump, self.vtune)?)?