Add inline version of duplicate

[Emoun]

Introduce a function-like procedural macro with the same functionality as duplicate.

duplicate can only be used where attributes are allowed. Notably, attributes are currently not allowed on statements. This is a significant limitation. Therefore, we can alleviate this by introducing a function-like version of duplicate that can be used where the original is not allowed.

Example:

fn main() {
  duplicate_inline!{
    #[
      name    value; 
      [var1]  [1usize]; 
      [var2]  [2usize]; 
      [var3]  [2usize]
    ]
    let name = value;
  }
}

Expands to:

fn main() {
    let var1 = 1usize;
    let var2 = 2usize;
    let var3 = 3usize;
}

[Emoun]

Initial implementation done. Though the syntax doesn't use a # before the invocation block.

[Emoun]

Release as part of v0.2.7