Fix CIR parsing problem with struct/classes

[bcardosolopes]

For something simple as:

struct String {
  long size;
  long capacity;

  String() : size{0}, capacity{0} {}
  String(char const *s) : size{strlen(s)}, capacity{size} {}
};

We can emit CIR but not read it back:

error: expected '=' in type alias definition
!22struct2EString22 = !cir.struct<"struct.String", i64, i64>
     ^

We probably need some CIR specific prefix here or something.

[cmarcelo]

PRs #22 and #23 will fix the key issue with roundtrip here. Tested with an example similar but without the strlen call.