new syntax: pin x = e1 in e2

[vekatze]

Syntax:

pin x = e1 in
e2

Semantics:

pin x = e1 in
e2

↓

let x = e1 in
let result on x = e2 in
let _ = x in
result

Example:

// before
define foo(): unit {
  let xs = make-list(123) in
  let result on xs = some-func(xs) in
  let _ = xs in
  result
}

↓

// after
define foo(): unit {
  pin xs = make-list(123) in
  some-func(xs)
}