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optimize

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optimize(tool): 优化了生成工具关于 frugal 替换参数的实现逻辑

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en:
Previously: gen-frugal was used to decide whether to replace the implementation of structs with frugal based on annotations, and -frugal-struct xxx was used to pass the struct name when executing a command.

Now: Only the -frugal-struct parameter is retained, with different inputs achieving different effects:

1. -frugal-struct @ALL: Replaces all structs named fastCodec with frugal implementation.
2. -frugal-struct @auto: Replaces structs with annotations (equivalent to the previous gen-frugal parameter).
3. -frugal-struct xxxname: Specifies a specific struct, can be specified multiple times.

And also some template optimizations:

• implement BLength() by frugal EncodedSize.
• remove fastReadField, fastWriteField, fieldLength function because it's no longer used after frugal replacement.

zh(optional):
把 -gen-frugal 和 -frugal-struct 合并了

之前：有 gen-frugal 来决定是否根据注解给结构体替换 frugal 实现，也有 -frugal-struct xxx 执行命令行时传入结构体名来替换
现在：只保留 -frugal-struct 参数，不同输入达到不同效果：

1. -frugal-struct @ALL：给所有结构体 fastCodec 都替换为 frugal 实现
2. -frugal-struct @auto: 给有注解的结构体替换（相当于之前的gen-frugal参数）
3. -frugal-struct xxxname: 指定具体结构体，可以多次指定，和之前一样

模板上也有一些改动：

1. BLength 也改为用 frugal EncodedSize 来替换实现了
2. 开启 frugal 替换后，不再生成 fastReadField, fastWriteField, fieldLength，因为用不到了

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