feat: Add back to real env implementation

[winston-h-zhang]

This PR adds back to the original env implementation, along with the env memory.

• I ran into conflicts between the "nil env" and nil itself, so I opted to use a 'nil-env symbol instead, and manually check for it.
• Slight changes have been made to match with the current Lurk implementation (e.g. allocatiing a "nil env ptr" at address 0).