* cannon: Implement MIPS64Memory.sol

* cannon: Implement 64-bit Solidity VM

- Implements 64-bit Cannon (with multithreading) in MIPS64.sol
- Re-enable differential testing for 64-bit VMs

* review comments

* check pc for 4-byte alignment

* gofmt

* update snapshot

* address nits; add more add/sub/mult overflow tests

* diff test misaligned instruction

* fix mul[t] MIPS64.sol emulation

* diff fuzz mul operations

* fix addiu test case

* fix GetInstruction return value type