Function definition, declaration, and call

[Larsouille25]

Tracking issue for RFC-0001.

Todo

• [x] function definition
  • [x] func def with ABI
• [x] function declaration
• [x] function call
• [x] return statement
• [x] name mangling -> ZomIR

[Larsouille25]

This RFC is fully implemented, so it's closed