FFI

[giann]

FFI implementation. Inside:

• New zdef statement to declare foreign functions and structs in the global scope
• New std lib ffi
• FFI calls are generated with MIR
• Foreign struct getter/setters are generated with MIR
• New Buffer method to handle foreign types:
  • writeZ, writeZAt
  • readZ, readZAt
  • writeStruct, writeStructAt
  • ptr
• https://github.com/buzz-language/buzz/issues/2
• https://github.com/buzz-language/buzz/issues/21
• https://github.com/buzz-language/buzz/issues/138
• https://github.com/buzz-language/buzz/issues/165