[feat] make assembly block Solidity memory safe

[jonathanpwang]

Previously the assembly block for snark verifier started mstore from memory offset 0. This does not respect Solidity's memory layout where 0x40 = 64 is reserved as a free memory pointer. See https://docs.soliditylang.org/en/latest/assembly.html#memory-safety

We make the assembly block memory safe by changing EvmLoader::new to start ptr from MEM_PTR_START = 0x80, and check in the assembly block preamble that mload(0x40) = 0x80. This means that the memory between 0 and 0x80 is never used.

@han0110 The one thing I was a little unsure of was the common_scalar in EvmTranscript: is common_scalar only called exactly once at the very beginning on the transcript_initial_repr and so it's special case mload to 0x80 (formerly 0x00)?

[han0110]

LGTM!

The one thing I was a little unsure of was the common_scalar in EvmTranscript: is common_scalar only called exactly once at the very beginning on the transcript_initial_repr and so it's special case mload to 0x80 (formerly 0x00)?

Yes exactly, it's preserved for transcript_repr (it's a stupid design and should be fix tho..)