Remove zeroio

[shkoo]

Zeroio is no longer as big of a win over risc0_zkvm::serde as it was since our I/O works differently now. Plus, it was cumbersome to use.

[github-actions]

Benchmark for Linux-cuda dd083bb

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Test	Base	PR	%
fib/100/proof	819.3±39.20ms	815.9±53.89ms	-0.41%
fib/100/run	340.7±10.27ms	338.0±6.34ms	-0.79%
fib/200/proof	844.2±46.06ms	835.8±54.54ms	-1.00%
fib/200/run	343.1±9.36ms	319.8±14.08ms	-6.79%

Benchmark for Linux-default dd083bb

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Test	Base	PR	%
fib/100/proof	2.3±0.02s	2.3±0.02s	0.00%
fib/100/run	331.8±5.49ms	324.1±11.23ms	-2.32%
fib/200/proof	2.3±0.01s	2.3±0.01s	0.00%
fib/200/run	335.6±5.76ms	328.7±5.28ms	-2.06%

Benchmark for macOS-default dd083bb

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Test	Base	PR	%
fib/100/proof	1610.0±30.14ms	1603.5±25.99ms	-0.40%
fib/100/run	110.9±0.60ms	110.5±0.47ms	-0.36%
fib/200/proof	1601.5±10.05ms	1589.1±7.53ms	-0.77%
fib/200/run	112.7±0.80ms	112.3±0.59ms	-0.35%

Benchmark for macOS-metal dd083bb

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Test	Base	PR	%
fib/100/proof	470.0±5.28ms	465.2±4.61ms	-1.02%
fib/100/run	117.5±0.38ms	113.4±1.18ms	-3.49%
fib/200/proof	470.5±4.83ms	468.6±3.14ms	-0.40%
fib/200/run	119.6±0.33ms	115.5±0.93ms	-3.43%