Merge #601

601: union support for hir type checking and gcc backend r=dkm a=dkm

From Mark Wielaard : https://gcc.gnu.org/pipermail/gcc-rust/2021-August/000107.html

> 
> Treat a union as a Struct variant like a tuple struct.  Add an
> iterator and get_identifier functions to the AST Union class.  Same
> for the HIR Union class, plus a get_generics_params method. Add a new
> ADTKind enum and adt_kind field to the ADTType to select the
> underlying abstract data type (struct struct, tuple struct or union,
> with enum as possible future variant).
> 
> An union constructor can have only one field. Add an union_index field
> to StructExprStruct which is set during type checking in the
> TypeCheckStructExpr HIR StructExprStructFields visitor.
> 
> For the Gcc_backend class rename fill_in_struct to fill_in_fields and
> use it from a new union_type method. Handle union_index in
> constructor_expression (so only one field is initialized).

Fixes #157 

Co-authored-by: Mark Wielaard <mark@klomp.org>

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