Add `dec` and `dec_length` to `ProgressBar` #984

Job	Run time
audit	17s
lint	23s
cross (armv5te-unknown-linux-gnueabi)	33s
msrv	19s
test (ubuntu-latest, stable, --all-features)	28s
test (macos-latest, stable, --all-features)	24s
test (windows-latest, stable, --all-features)	2m 32s
test (ubuntu-latest, stable)	26s
test (ubuntu-latest, beta, --all-features)	27s
test (ubuntu-latest, stable, --all-features, --target armv5te-unknown-linux-gnueabi, true)	30s
	6m 19s

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