Disallow splices in right-hand side of `let rec`

[OlivierNicole]

We want to avoid:

# macro m x = << let rec g = $(x << g >>) in g >>;;
macro m : ('a expr -> 'a expr) -> 'a expr = <fun>
# m (fun y -> y);;;
- : 'a expr = << let rec g_2 = g_2  in g_2 >