Chirp `let` statements

[nicopap]

Similarly to #80, we'd like a way to define templates. A quicker-to-implement objective would be "template without argument".

let template = MyEntity(row) {
  Image(ui("image.png") width(100%) color(coral))
}

Similarly to fn, we could use use:

use template

[nicopap]

I'm not interested in let statements. They are equivalent to fn statements with no parameters.