[mypyc] Tweak how tuple structs are defined

[msullivan]

Change how we declare and define tuple structs to prepare for separate compilation:

• Wrap declarations in an #ifdef/#define so that multiple modules
  can define the same tuple types (since they are structural) and
  interoperate properly.
• Declare the undefined values eagerly instead of only when they
  are needed, since they might be needed by another compilationg group.
• Declare them with a typedef instead of always writing struct. This
  will very slightly simplify some logic for manipulating the type string
  later but mostly just looks better.