Tweaks to promises #905

Job	Run time
bindings-windows (x86_64-pc-windows-msvc, true)	6m 58s
conformance	11m 13s
bindings-linux (x86_64-unknown-linux-gnu)	2m 28s
bindings-windows (i686-pc-windows-msvc, false)	4m 39s
bindings-macos (x86_64-apple-darwin)	2m 21s
bindings-linux (i686-unknown-linux-gnu)	2m 32s
bindings-linux (x86_64-unknown-linux-musl)	2m 41s
documentation	10m 23s
bindings-linux (arm-unknown-linux-gnueabihf)	2m 29s
guide	1m 11s
bindings-linux (arm-unknown-linux-musleabihf)	2m 51s
features (--no-default-features)	1m 45s
bindings-linux (aarch64-unknown-linux-gnu)	2m 32s
features (--no-default-features --features serialization)	1m 49s
linting (stable)	1m 3s
bindings-linux (aarch64-unknown-linux-musl)	2m 57s
test (ubuntu-latest, stable)	1m 34s
linting (beta)	55s
features (--no-default-features --features serial)	1m 44s
bindings-linux (armv7-unknown-linux-gnueabihf)	2m 34s
test (ubuntu-latest, beta)	1m 28s
features (--no-default-features --features tls)	1m 53s
bindings-linux (arm-unknown-linux-gnueabi)	2m 42s
test (windows-latest, stable)	3m 4s
test (windows-latest, beta)	3m 16s
packaging	3m 36s
release	0s
	1h 22m 38s

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