Update fs_at requirement from 0.1.4 to 0.2.1 #110

Job	Run time
build-unix (stable, ubuntu)	15s
build-windows (i686, msvc, stable)	2m 57s
build-unix (stable, macos)	13s
build-windows (i686, msvc, beta)	1m 3s
build-unix (beta, ubuntu)	22s
build-windows (i686, msvc, nightly)	1m 4s
build-unix (beta, macos)	19s
build-windows (x86_64, gnu, stable)	5m 11s
build-unix (nightly, ubuntu)	17s
build-windows (x86_64, gnu, beta)	3m 19s
build-unix (nightly, macos)	22s
build-windows (x86_64, gnu, nightly)	3m 11s
build-windows (x86_64, msvc, stable)	1m 52s
build-windows (x86_64, msvc, beta)	48s
build-windows (x86_64, msvc, nightly)	53s
build-windows (aarch64, msvc, stable)	1m 27s
build-windows (aarch64, msvc, beta)	39s
build-windows (aarch64, msvc, nightly)	35s
	24m 47s

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