This crate export a macro [`Struct`] to instantiate a struct with others which have common
fields, the usage is similar to struct update syntax, but allow to different types and must
provide field names.
## Usage
```usage
let a = Struct! { StructA,
    field: value,
    ...
    [field2, field3, ...]: other,
    ...
};
```
which expand to:
```usage
let a = StructA {
   field: value,
   ...
   field2: other.field2,
   field3: other.field3,
   ...
};
```
## Example
``` rust
struct A {
    f1: i32,
    f2: &'static str,
    f3: i64,
    f4: &'static str,
    f5: u64,
}
struct B {
    f3: i64,
    f4: &'static str,
}
struct C {
    f5: u64,
}

let b = B { f3: 5, f4: "tt" };
let c = C { f5: 7 };

use struct_update::Struct;
let a = Struct! {A,
            f1: 3 + 5,
            f2: "sss",
            [f3, f4]: ..b,
            [f5]: ..c,
        };
assert_eq!(a.f1, 8);
assert_eq!(a.f2, "sss");
assert_eq!(a.f3, b.f3);
assert_eq!(a.f4, b.f4);
assert_eq!(a.f5, c.f5);
```

## Workaroud for macro path
Pitifully, in macro context we cannot use path(::) to instantiate a struct, the workaround is
converting a path to an ident.
### Usage
The `as _Struct` is optional, `_Struct` is the default name.
```usage
let a = Struct! { a_crate::a_mod::StructA as _Struct,
    field: value,
    ...
    [field2, field3, ...]: other,
    ...
};
```
which expand to:
```usage
let a = {
    use a_crate::a_mod::StructA as _Struct;
    _Struct {
       field: value,
       ...
       field2: other.field2,
       field3: other.field3,
       ...
    }
};
```