deps: update libflate requirement from 1.0.0 to 2.1.0 #574

Job	Run time
documentation	22s
clippy	21s
rustfmt	13s
test (msrv, ubuntu-20.04, 1.64.0)	44s
benchmarks	43s
test (stable, ubuntu-20.04, stable)	28s
Miri	40s
Optional features (approx selected)	31s
test (beta, ubuntu-20.04, beta)	45s
test (nightly, ubuntu-20.04, nightly)	41s
Optional features (none selected)	26s
test (macos, macos-latest, stable)	47s
static SuiteSparse (stable, ubuntu-20.04, stable)	26s
test (win-msvc, windows-2019, stable)	31s
static SuiteSparse (macos, macos-latest, stable)	47s
test (win-gnu, windows-2019, stable)	28s
static SuiteSparse (win-msvc, windows-2019, stable)	11s
static SuiteSparse (win-gnu, windows-2019, stable)	31s
	9m 35s

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