Commits on Jul 24, 2024

1. Change AST for iterations to use iteration kind …

   The `=` node which has traditionally been used for iteration
   specifications in `for` loops and generators doesn't have normal
   assignment semantics. Let's consider
   
       for x in xs
           body
       end
   
   which has been parsed as `(for (= x xs) (block body))`.
   
   Problems:
   * The iteration does create a binding for `x`, but not to the expression
     on the right hand side of the `=`.
   * The user may use `in` or `∈` in the source code rather than `=`.
     The parser still uses a `=` node for consistency but this only
     emphasizes that there's something a bit weird going on.
   
   So this use of `=` is not assignment; merely assignment-like.
   
   In this change, we use `in` instead of `=` and wrap this in an
   `iteration` node so that all iteration (including over multiple
   iterators) has the same structure.
   
   Thus the `for` loop above parses as `(for (iteration (in x xs)) (block body))`
   instead.
   
   The `cartesian_iteration` head naturally becomes `iteration` instead -
   being less specific here with the naming seems appropriate in trying to
   represent the surface syntax; cartesian semantics come later in lowering
   and a macro may decide to do something else with the iteration spec.
   
   These changes are also used for generators.
   
   After the changes we have tree structures such as
   
       julia> parsestmt(SyntaxNode, "for i in is body end")
       line:col│ tree                                   │ file_name
          1:1  │[for]
          1:4  │  [iteration]
          1:4  │    [in]
          1:5  │      i
          1:10 │      is
          1:12 │  [block]
          1:13 │    body
   
       julia> parsestmt(SyntaxNode, "for i in is, j in js body end")
       line:col│ tree                                   │ file_name
          1:1  │[for]
          1:4  │  [iteration]
          1:4  │    [in]
          1:5  │      i
          1:10 │      is
          1:13 │    [in]
          1:14 │      j
          1:19 │      js
          1:21 │  [block]
          1:22 │    body
   
       julia> parsestmt(SyntaxNode, "[a for i = is, j = js if z]")
       line:col│ tree                                   │ file_name
          1:1  │[comprehension]
          1:2  │  [generator]
          1:2  │    a
          1:7  │    [filter]
          1:7  │      [iteration]
          1:7  │        [in]
          1:8  │          i
          1:12 │          is
          1:15 │        [in]
          1:16 │          j
          1:20 │          js
          1:26 │      z
   
       julia> parsestmt(SyntaxNode, "[a for i = is for j = js if z]")
       line:col│ tree                                   │ file_name
          1:1  │[comprehension]
          1:2  │  [generator]
          1:2  │    a
          1:7  │    [iteration]
          1:7  │      [in]
          1:8  │        i
          1:12 │        is
          1:18 │    [filter]
          1:18 │      [iteration]
          1:18 │        [in]
          1:19 │          j
          1:23 │          js
          1:29 │      z

   

   c42f committed Jul 24, 2024
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