C gen: make variants unboxed

[osa1]

In prep for value types, this makes variant types unboxed.

For example, [E1, E2, E3] generates this C struct:

typedef struct Variant_E1_E2_E3 {
    uint64_t _tag;
    union {
        E1* _0;
        E2* _1;
        E3* _2;
    } _alt;
} Variant_E1_E2_E3;

This struct is then used directly, instead of as a pointer.

The tag is the tag of the variant's wrapped value. E.g. if the variant value is
E1, the tag is the tag of E1. If E1 is a sum type, then it's the tag of
the constructor.

(Because each constructor (rather than type) has a unique constructor, we can
use the constructor's type as variant tags.)

This means that we always check the tag once when matching a variant value,
regardless of the values in the variant. (sums or products, boxed or unboxed)

Variants are still assigned heap object indices, but those indices are not used
in runtime. They're used in compile time to refer to variant types during
dependency analysis (and probably other places too).

Variant type refinements: we now annotate AST nodes with refined types of
variant binders. The generated code then unpacks the original value and packs
again as the value expected by the binder. Example:

type E1
type E2
type E3

test(x: [E1, E2, E3]) [E2, E3]:
    match x:
        ~E1: panic("")
        other: other

Compiled to: (relevant parts)

else if (({
   _1 = ({
     Variant_E2_E3 _t6;
     if ((_t4)._tag == TAG_E2) {
       _t6._tag = TAG_E2;
       _t6._alt._0 = (_t4)._alt._1;
     } else if ((_t4)._tag == TAG_E3) {
       _t6._tag = TAG_E3;
       _t6._alt._1 = (_t4)._alt._2;
     } else {
       fprintf(stderr, "Invalid variant conversion\n");
       exit(1);
     }
     _t6;
   });
   1;
 })) {
  ...
}

This part of the generated code is not properly indented/formatted yet, example
above formatted manually with clang-format.

We should generate variant conversions outlined, in a separate function.

The "invalid variant conversion" is there to make debugging easier. That branch
won't be taken unless there's a bug in the compiler.

Fixes #279.