Expr is too big

[kdy1]

I found one serious problem of SWC ES AST, which wastes enormous amount of memory / performance.

The problem is that, Expr was needlessly 96 bytes. I optimized it by boxing some large variants, but while refactoring in the past, AssignExpr -> AssignTarget got too big without being caught in a code review.

(Sadly, static assertion was disabled due to a layout size bug of old rustc)
// Memory layout depends on the version of rustc.
// #[cfg(target_pointer_width = "64")]
// assert_eq_size!(Expr, [u8; 80]);

I'll work on this today.