fix default_alloc import error

[Cupnfish]

default_alloc宏在内部递归了自己的调用，但是现有的实现疏忽了$crate，这导致使用ckb_std::default_alloc!()的时候报错了

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The default_alloc macro recursively calls itself internally, but the existing implementation overlooked the $crate identifier. This oversight leads to an error when using ckb_std::default_alloc!().

[Cupnfish]

我正在实现一个基于ckb-std二次开发的包，这个修复对我很重要，希望早日能够合并到主分支。

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I am currently developing a package based on ckb-std for customization, and this fix is of great importance to me. I hope it can be merged into the main branch promptly.