Switch to using half::f16 #95

Job	Run time
Test full	20s
windows-latest	3m 16s
macos-latest	44s
Clippy	45s
cross (wasm32-unknown-unknown)	34s
Feature coverage	3m 45s
cross (i686-unknown-linux-gnu)	1m 2s
fmt	29s
cross (powerpc-unknown-linux-gnu)	1m 0s
SIMD	52s
cross (powerpc64-unknown-linux-gnu)	1m 9s
Miri	17m 49s
MIRI on IO IPC	55s
cross (arm-linux-androideabi)	1m 12s
MIRI on IO IPC mmaping	55s
	34m 47s

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