Improve let-else formatting

[dtolnay]

Fixes #55.

The following styles are all mirrored from rustfmt.

impl op_v8_string {
    extern "C" fn v8_fn_ptr(info: *const deno_core::v8::FunctionCallbackInfo) {
        let result = {
            let Ok(arg0) = v8_convert(arg0) else { return };
            let Ok(mut arg0) = deno_core::_ops::v8_try_convert::<T>(arg0) else {
                let mut scope = unsafe { deno_core::v8::CallbackScope::new(&*info) };
                return;
            };
            let Ok(mut arg0) = deno_core::_ops::v8_try_convert::<deno_core::String>(arg0)
            else {
                let mut scope = unsafe { deno_core::v8::CallbackScope::new(&*info) };
                return;
            };
            let Ok(mut arg0) = deno_core::_ops::v8_try_convert::<
                deno_core::v8::String,
            >(arg00000000000000000000000000) else {
                let mut scope = unsafe { deno_core::v8::CallbackScope::new(&*info) };
                return;
            };
        };
    }
}