More consistent let/letrec syntax

[rolyp]

Drop the fun keyword from letrec, so we have:

letrec
   f x y → ...;
   g a b → ...

Then similarly generalise the let syntax, allowing:

let f x y → ...;
let g a b → ...

There is no case for having a letrec analogue of the “first-class function” notation for let, since that would suggest that arbitrary recursive values are permitted:

letrec 
   f = fun x y → ...;
   g = fun a b →

Experimented with desugaring into a syntax such as the above as part of #225. Also consider splitting letrec into two keywords, a la O'Caml.

• traditional let x = e in e’ instead of prior "defs" approach , but write in as ;
• let rec
  • drop rec, and assume all named functions recursive
  • modules as sequences of let and recursive let definitions
  • allow -> as well as = for consistency with other eliminator occurrences
  • allow blocks of recursive definitions
• non-recursive (anonymous) functions
  • syntax, semantics and slicing
  • parsing

[rolyp]

Doing as part of #281.