The srcloc in grift0 is shifted for bingind structures with only one sub-expression in body. #116

The bug is caused by the parsing algorithm in src/grift/syntax->grift0.rkt.

For example, consider the following Grift program

(let ([a 1])
  (+ a 2))

The syntax->grift0 will give the following AST:

(Prog
 '("bad-let-2.grift" 1)
 (list
  (Ann2
   (Observe
    (Ann
     (Let
      (list
       (Bnd
        (Uid "a" 0)
        #f
        (Ann (Quote 1) (srcloc "bad-let-2.grift" 1 9 10 1))))
      (Ann
       (Op
        '+
        (list
         (Ann (Var (Uid "a" 0)) (srcloc "bad-let-2.grift" 2 5 19 1))
         (Ann (Quote 2) (srcloc "bad-let-2.grift" 2 7 21 1))))
       (srcloc "bad-let-2.grift" 1 0 1 23)))
     (srcloc "bad-let-2.grift" 1 0 1 23))
    #f)
   (srcloc "bad-let-2.grift" 1 0 1 23))))

The problem is that, the srcloc for (let ([a 1]) (+ a 2)) is exactly the srcloc for (+ a 2). Both of them are (1 0 1 1 23).

In general, the description of this bug is: if let or let-rec only has one expression in its body, then the srcloc of this expression will be the same as the outer let or let-rec.
I believe that is because of the weird logic of parse-begin. It seems that when the body of (begin e) only has one expression, this parsing algorithm will omit the outer begin. And all let, let-rec, define transform their bodies into a begin.