fix: Memory edge case in pad & padMemory

[clabby]

Overview

Fixes an edge case in LibKeccak's pad / padMemory functions, where it was possible for dirty bits to enter the zero'd out padding space. This is due to a detail of solidity's memory safety considerations, where it may leave certain data after the free memory pointer dirty at times so that the memory can be reused without expanding further, for example when it abi encodes the calldata for an external call (contract.method(params) syntax):

How it's fixed

We now manually clean the memory inbetween the padding bytes that we store, just like solc would.

Testing

• expectSafeMemory tests for the padding functions. These assert that no memory is writtten to out of bounds of the expected range.
• e2e pad -> absorption -> permutation -> squeeze process, with brutalized memory, thanks to @Vectorized's brutalizeMemory helper. This directly reproduces the bug found while fuzzing in the monorepo on the older version, and asserts that the full flow is safe when allocating in non-clean memory.