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.

build.rs

@@ -30,6 +30,7 @@ fn main() -> anyhow::Result<()> {
             test_directory(out, "tests/misc_testsuite", strategy)?;
             test_directory_module(out, "tests/misc_testsuite/bulk-memory-operations", strategy)?;
             test_directory_module(out, "tests/misc_testsuite/reference-types", strategy)?;
+            test_directory_module(out, "tests/misc_testsuite/multi-memory", strategy)?;
             Ok(())
         })?;