Misc. improvements

• Emit non-zero array constants properly
• Emit only actually used array constants per runtime wrapper
• Juno CLI
• Use repr(C) structs for product types
• Minimal implementation of SROA
• TODO: inlining, codegen cast, intrinsics, dynamic constant math

juno_frontend/examples/simple1.jn

+fn simple1(x : i32, y : i32) -> i32 {
+  return x + y;
+}