Modularize mock crate for easier and more customizable testing setup generation

[CPerezz]

Adds a Builder pattern for constructing almost any test case and context that is required.

This is an example on how to use it:

let code_a = bytecode! {
    PUSH1(0x0) // retLength
    PUSH1(0x0) // retOffset
    PUSH1(0x0) // argsLength
    PUSH1(0x0) // argsOffset
    PUSH32(Word::from(0x1000)) // value
    PUSH32(*WORD_ADDR_B) // addr
    PUSH32(0x1_0000) // gas
    CALL

    PUSH2(0xaa)
};
let code_b = bytecode! {
    PUSH1(0x01) // value
    PUSH1(0x02) // key
    SSTORE

    PUSH3(0xbb)
};

// Get the execution steps from the external tracer
let block: GethData = TestContext::<3, 2>::new(
    None,
    |accs| {
        accs[0]
            .address(address!("0x0000000000000000000000000000000000000000"))
            .code(code_a);
        accs[1]
            .address(address!("0x000000000000000000000000000000000cafe001"))
            .code(code_b);
        accs[2]
            .address(address!("0x000000000000000000000000000000000cafe002"))
            .balance(Word::from(1u64 << 30));
        accs
    },
    |mut txs, accs| {
        txs[0].to(accs[0].address).from(accs[2].address);
        txs[1]
            .to(accs[1].address)
            .from(accs[2].address)
            .nonce(Word::one());
    },
    |block, _tx| block.number(0xcafeu64),
)
.unwrap()
.into();

Here we can see that works with const-generics and we need to specify: NACC (Number of accounts) and NTX (Number of transactions).

TODO:

• Fix EXTCODEHASH testcases that are not passing. https://github.com/appliedzkp/zkevm-circuits/blob/5d8166ddfa5060b7c3d515ae2a74bd8cb7024bc7/bus-mapping/src/evm/opcodes/extcodehash.rs#L145-L154
• Figure out with @han0110 what to do with some tests with excessive gas_price which panic. https://github.com/appliedzkp/zkevm-circuits/blob/5d8166ddfa5060b7c3d515ae2a74bd8cb7024bc7/zkevm-circuits/src/evm_circuit/execution/begin_tx.rs#L399-L430
• Add some documentation.
• Final cleanup (if required)

Resolves: #348

[CPerezz]

This will remain untouched until #312 gets merged so that no duplicated work/conflicts increase.

[CPerezz]

@ed255 it's ready for the final review! :)

[han0110]

LGTM! The builder pattern makes everything easier to understand! Only got some nitpicks.

[ed255]

I've completed my review! I've left a few notes and comments, please take a look :)

[ed255]

LGTM! Awesome work!

[han0110]

LGTM!