Syntax for splicing let form in lambda bodies

[vyzo]

I propose this syntax:

(def (string-substitute-char ...)
  (let key =: key) ; notice the keyword to make the discriminant
  ...)

Originally posted by @vyzo in https://github.com/vyzo/gerbil/pull/826#discussion_r1325611962

[fare]

Nice syntax, but... what's the semantics?