Rename sub code generater emit -> gen
original commit by miura1729
https://github.com/miura1729/mruby/commit/b20dd48b6b6e601f6de5222d20e9df7226847b46
NaN Boxing
Add type guard
original commit by miura1729
https://github.com/miura1729/mruby/commit/1d4cf4581d10ec066a0937f15de97b543f34d948
change Regs Map
remove debug code
fix loadl bug.
original fix is 1bf72656e1b38f3b84687ab78261f25ee1b25d46
ignore jitcode.h if not defined ENABLE_JIT
Add overflow check
original commit by miura1729
https://github.com/miura1729/mruby/commit/5dbacb80ee2e62b441aabd147b93680463cbb007