Rust: rename MatchExpr.expr to scrutinee in all layers

This doesn't require `ql.name` and is simpler while we don't have
to write upgrade scripts. The `ql.name` mechanism might get useful
once we do have to write upgrade scripts, as that doesn't change the
dbscheme.

rust/schema/ast.py

@@ -364,7 +364,7 @@ class MatchArmList(AstNode):
 
 class MatchExpr(Expr):
    attrs: list["Attr"] | child
-   expr: optional["Expr"] | child
+   scrutinee: optional["Expr"] | child
    match_arm_list: optional["MatchArmList"] | child
 
 class MatchGuard(AstNode):