Rust: distinguish [a, b] from [a; b]

This splits the `ArrayExpr` class into `ArrayListExpr` and `ArrayRepeatExpr`.
This uses the `synth.from_class` machinery to integrate seamlessly into the
generated code, by hiding the extracted `ArrayExpr` behind an internal class
and replacing it with a hierarchy of those two classes under a new
`ArrayExpr` class.

rust/schema/ast.py

@@ -38,9 +38,10 @@ class Abi(AstNode):
 class ArgList(AstNode):
    args: list["Expr"] | child
 
-class ArrayExpr(Expr):
+class ArrayExprInternal(Expr):
    attrs: list["Attr"] | child
    exprs: list["Expr"] | child
+   is_semicolon: predicate
 
 class ArrayType(TypeRef):
    const_arg: optional["ConstArg"] | child