Switch to using half::f16 #93

Job	Run time
Clippy	3m 59s
cross (wasm32-unknown-unknown)	30s
cross (i686-unknown-linux-gnu)	57s
Feature coverage	3m 29s
cross (powerpc-unknown-linux-gnu)	56s
fmt	23s
cross (powerpc64-unknown-linux-gnu)	1m 7s
cross (arm-linux-androideabi)	1m 14s
SIMD	53s
Miri	17m 26s
MIRI on IO IPC	1m 5s
Test full	19s
windows-latest	3m 22s
MIRI on IO IPC mmaping	55s
macos-latest	47s
	37m 22s

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