Add an "umbrella" crate, which seeks to provide a convenient interface.

This currently pulls in cretonne-codegen and cretonne-frontend and
provides a simple prelude interface.

This fixes #287.

cranelift/publish-all.sh

@@ -27,7 +27,13 @@ cargo update
 
 echo git commit -a -m "\"Bump version to $version"\"
 echo git push
-for crate in entity codegen frontend native reader wasm ; do
+for crate in entity codegen frontend native reader wasm umbrella ; do
+    if [ "$crate" == "umbrella" ]; then
+        dir="cretonne"
+    else
+        dir="$crate"
+    fi
+
     echo cargo publish --manifest-path "lib/$crate/Cargo.toml"
 done
 echo