Parser: parameterize `let_binding` and `let_bindings` with `EXT`. `EXT` can be instantiated with either `ext` of `no_ext`. This allows getting rid of one use of `not_expecting` in the auxiliary function `class_of_let_bindings`. This use of `not_expecting` is now clearly isolated in the definition of `no_ext`.
Parser: parameterize `let_binding` and `let_bindings` with `EXT`.
`EXT` can be instantiated with either `ext` of `no_ext`.
This allows getting rid of one use of `not_expecting`
in the auxiliary function `class_of_let_bindings`.
This use of `not_expecting` is now clearly isolated
in the definition of `no_ext`.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml