Outstanding TODO comments

[tcrypt25519]

Tracks all outstanding TODO comments found across the crates, grouped by theme.

Child issues

• Re-enable virtual stack (vstack) optimization #33 — Re-enable virtual stack (vstack) optimization
• Implement EXP and SIGNEXTEND opcodes #34 — Implement EXP and SIGNEXTEND opcodes
• keccak256: check return code and optimize stack push #35 — keccak256: check return code and optimize stack push
• Optimize POP and RETURN opcode codegen #36 — Optimize POP and RETURN opcode codegen
• Implement dynamic memory expansion and MSTORE memory tracking #37 — Implement dynamic memory expansion and MSTORE memory tracking
• Add bounds checking to runtime stack_pop and stack_peek #38 — Add bounds checking to runtime stack_pop and stack_peek