Prepare 0.15.3 release #828

Job	Run time
linux-check-and-test-armv7	1m 34s
rustfmt-check	9s
clippy-test	42s
ubuntu-test	38s
android-apk-build	1m 36s
android-check	35s
asmjs-wasm32-test (wasm32-unknown-emscripten)	46s
cargo-publish	0s
ios-build	2m 45s
macos-test	1m 28s
wasm32-bindgen-test (wasm32-unknown-unknown)	33s
wasm32-wasi-test (wasm32-wasi)	13s
wasm-beep-build	28s
windows-test (x86_64)	2m 8s
windows-test (i686)	3m 15s
	16m 50s

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