A macro to generating AST with python grammar

Inspired by https://docs.rs/pmutil/latest/pmutil/macro.smart_quote.html

e.g.

ExprKind::Tuple(ast::ExprTuple {
    elts: names
        .iter()
        .map(|&name| {
            create_expr(ExprKind::Constant(
                ast::ExprConstant {
                    value: Constant::Str(name.to_string()),
                    kind: None,
                },
            ))
        })
        .collect(),
    ctx: ExprContext::Load,
}))

can be rewritten to:

&python_ast! {
    Vars { names },
    "(*names)"
}

The grammar will be limited to legal python codes to leverage python parser.

Start by reading the referenced pmutil smart_quote macro and the proposed python_ast! invocation in this issue. Investigate the parser and AST-generation entry points in RustPython/Parser before deciding how the Python grammar should be constrained. Done means the macro can turn legal Python snippets plus supplied bindings into the corresponding AST.