SSTORE and SLOAD fuzzing

This implements a fuzzer targeting SSTORE and SLOAD, to test out the implementations of EIP-2200 (EIP-1283).

Fuzzing scheme

The fuzzer create a set of accounts (10) ([addrs]), and initializes each with random code. The code generated by randcode will randomly select from the following actions

// 30% sstore,
// 30% sload,
// 20% call of some type (CALL, CALLCODE, STATIC etc), to one of addrs
// 5% create, 5% create2,
// 5% return, 5% revert

A CREATE/CREATE2 + CALL(addr), will have init-code that does

// 40% chance of sstore,
// 40% chance of sload
// 20% chance of nothing more

and then deploys bytecode generated as outlined above.

The idea is to set/reset slots from various contexts, newly created and not, reverted and not.