Auto merge of #1437 - sunfishcode:master, r=gnzlbg

Update wasmtime to the latest master.

The previous wasmtime revision is broken because one of its dependencies, `memoffset` 0.3, was yanked, which appears to make it unavailable. This was fixed by downgrading to `memoffset` 0.2.

Author: bors

ci/docker/wasm32-wasi/Dockerfile

@@ -60,7 +60,7 @@ ENV PATH=/root/.cargo/bin:$PATH
 RUN apt-get install -y --no-install-recommends python
 RUN git clone --recursive https://github.com/CraneStation/wasmtime wasmtime && \
   cd wasmtime && \
-  git reset --hard 67edb00f29b62864b00179fe4bfa99bc29973285
+  git reset --hard a2647878977726935c3d04c05cabad9607ec7606
 RUN cargo build --release --manifest-path wasmtime/Cargo.toml
 
 # And finally in the last image we're going to assemble everything together.