[FOLLOW UP] Codegen

[Y-Nak]

This is a follow-up issue for the newly added codegen module in #676.

• Allow using complex type as a map key #711
• Allow using complex type as array element type #730 <- Allow using a complex type as an array element type #734
• Support returning complex structs from public functions #635 <- Support encoding/decoding nested struct types #729
• Allow nested struct type arguments in public functions #733 <- Support encoding/decoding nested struct types #729
• Implement safe math operations in std #718
• Cleanup leftover of old lowering crate #720 <- Remove lowering cruft from ingot/module items #727
• Update architecture.md to reflect mir/codegen changes (ref Add architecture.md doc (WIP) #607)

[sbillig]

Defunct