Add .cargo/config.toml with rustflags = -O to optimize test runtimes

[dconnolly]

Motivation

When writing tests for #2645 , I created some tests that took over 5 minutes to run. When run with the rustc -O flag, they take ~30 seconds to run.

Solution

Add a .cargo/config.toml config file to provide -O as the rust flags for all compilations for our builds by default.

Review

@jvff @teor2345

[teor2345]

Some of the jobs are slower, but they should be faster once our build cache has cached an optimised build.

[teor2345]

It looks like we're going over 75 minutes for Windows. That's because we're optimising a whole bunch of code we don't use much in the tests.

I'll try with optimisation level 1 now (-O means optimisation level 2, which is slower to build):
https://doc.rust-lang.org/rustc/codegen-options/index.html#opt-level

We can also restrict the optimisation to just the halo2 crate. But I'm not sure how we automatically optimise its dependencies as well.

https://doc.rust-lang.org/cargo/reference/config.html#profilenamepackagename

[teor2345]

The stateful tests will override these settings, because environmental variables take precedence over configs:
https://doc.rust-lang.org/cargo/reference/config.html#environment-variables

So they'll continue to build with the higher optimisation level.

[teor2345]

I don't think turning on optimisations is going to work. We've tried optimisation levels 1 & 2, and they both make the builds much longer.

Let's serialize the generated proofs instead.

We can use the Halo2Proof consensus serialization format, which is implemented here:

zebra/zebra-chain/src/primitives/proofs/halo2.rs

Lines 25 to 29 in 02526c3

	impl ZcashSerialize for Halo2Proof {
	fn zcash_serialize<W: io::Write>(&self, writer: W) -> Result<(), io::Error> {
	zcash_serialize_bytes(&self.0, writer)
	}
	}

Here's how we load data from a hex-encoded file in a lazy_static! block:

zebra/zebra-test/src/vectors/block.rs

Lines 246 to 248 in 02526c3

	pub static ref BLOCK_MAINNET_GENESIS_BYTES: Vec<u8> =
	<Vec<u8>>::from_hex(include_str!("block-main-0-000-000.txt").trim())
	.expect("Block bytes are in valid hex representation");

Then we can deserialize typed data using code like:

zebra/zebrad/src/components/mempool/tests/vector.rs

Lines 370 to 372 in 0960e4f

	let genesis_block: Arc<Block> = zebra_test::vectors::BLOCK_MAINNET_GENESIS_BYTES
	.zcash_deserialize_into()
	.unwrap();

The serialization should be the reverse of these steps, let me know if you have any trouble!