bsn: support field name shorthand

[cart]

Objective

bsn! should support Rust's "name shorthand syntax":

let value = 10;
Foo { value }

Solution

Add support, both for normal types and "props":

let value = 10;
bsn! {
  Foo { value }
}

let value = 10;
bsn! {
  @Foo { @value }  
}

This also changes generated internal bsn! variables to use __X formatting everywhere to avoid conflicts with user-defined variables (we were already using this strategy in some places ... this just uses it everywhere).

Testing

• added a test

[kfc35]

looks good to me, have to resolve conflicts

[ickshonpe]

The code generation doesn't match the equivalent explicit version, suppose value is a u32 field:

let value = 10u8;
bsn! {
  Foo { value: value }
}

works because it adds an into() but not with:

let value = 10u8;
bsn! {
  Foo { value }
}

which results in a type mismatch error.

[laundmo]

This should be added to docs (at least the syntax table) once #24464 is merged