ast: Fix IndexExpr

[benstigsen]

Try running this code:

import ast
import json

var tree = ast.parse('array[1] = v', '')
echo json.encode(tree, false)

What you'll see is the following:

[
  {
    "type": "ExprStmt",
    "expr": {
      "type": "=",
      "expr": {
        "type": "IndexExpr",
        "args": [
          {
            "type": "LiteralExpr",
            "value": "1"
          }
        ]
      },
      "value": {
        "type": "IdentifierExpr",
        "value": "v"
      }
    }
  }
]

As can be seen our identifier array (or whatever might be on the left hand side), is never included in the expression node.
This PR adds an expr parameter to the IndexExpr node. Now our tree looks like this:

[
  {
    "type": "ExprStmt",
    "expr": {
      "type": "=",
      "expr": {
        "type": "IndexExpr",
        "expr": {
          "type": "IdentifierExpr",
          "value": "array"
        },
        "args": [
          {
            "type": "LiteralExpr",
            "value": "1"
          }
        ]
      },
      "value": {
        "type": "IdentifierExpr",
        "value": "v"
      }
    }
  }
]

Oh, and there's also a small documentation addition for clib.named_struct() which I accidentally committed. 😅

[benstigsen]

I think there might be a couple of issues. There is no way to tell whether a LiteralExpr is a string or a number. Ideally the LiteralExpr should contain the quotes themselves if it's a string.

I'll go through it this week and try to fix the remaining parts of the ast module. Feel free to merge this now if you think it's okay, or we can wait until the other fixes.

[mcfriend99]

The climb comments are welcomed since it clarified the functions.

I would have preferred it as a separate PR, but I think I can still merge it.