Coq backend: handle constructors with bases (aka {field: expr, ..base} in rust)

[W95Psp]

For now, the base is just skipped:

struct Foo {
    a: u8,
    b: u8,
}
fn f(x: Foo) -> Foo {
    Foo { a: 3, ..x }
}

Gets translated as:

Record Foo_tFoo : Type :={
  b : int8;
  a : int8;
}.

Definition f (x : Foo_t) : Foo_t :=
  Build_Foo_t (@repr WORDSIZE8 3).

There's also, something wrong with field projectors, related to #132

[W95Psp]

@cmester0, is that still an issue or it's supported now?

[github-actions]

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[franziskuskiefer]

@cmester0 is this fixed by #987