Build wasm lib with -O3

There doesn't seem to be much code size difference, but there's
a big performance difference.

script/build-wasm

@@ -31,7 +31,7 @@ set -e
 
 web_dir=lib/binding_web
 exports=$(cat ${web_dir}/exports.json)
-emscripten_flags="-Os"
+emscripten_flags="-O3"
 minify_js=1
 force_docker=0
 
@@ -39,7 +39,7 @@ while [[ $# > 0 ]]; do
   case "$1" in
     --debug)
       minify_js=0
-      emscripten_flags="-s ASSERTIONS=1 -s SAFE_HEAP=1 -Os"
+      emscripten_flags="-s ASSERTIONS=1 -s SAFE_HEAP=1 -O0"
       ;;
 
     --help)