[Enhancement] Allow attributes, like format_read, to have inlined code

首先以当前和拟议的 DATE 示例作为行为参考，追踪 format_read 和 transform 属性是如何被解析和应用的。确定内联表达式如何接收 struct 字段，以及现有的具名函数如何继续获得支持；完成的标准是两种形式都能工作，而不需要名称唯一的辅助函数。

Currently, various attributes like format_read require a string containing a function name as the argument. A lot of the time, these functions seem to end up being a simple one-liner.

// == current ==
struct DATE
{
    u16 year;
    u8 month;
    u8 day;
}
[[
    format_read("read_DATE"),
    transform("transform_DATE"),
    ...
]];
fn read_DATE(auto v)
{
    return std::format("{}-{}-{}", v.year, v.month, v.day);
};
fn transform_DATE(auto v)
{
    return (v.year * 10000) + (v.month * 100) + v.day;
};

This adds a decent amount of boilerplate overhead, making the underlying data format specification a little more difficult to read (due to tool integration 'noise'). Although the function could be moved to a different part of the pattern file, to somewhat separate data format from integration, the function would then no longer be co-located with the data it's operating on.

I feel things could be cleaner / clearer, if the contents of that function were optionally able to be inlined. This also avoids needing the pattern creator to make sure the function is uniquely named, as well as having to access the data through a (named) parameter.

As a first pass suggestion, inlining simple one-liners could look like:

// == proposed ==
struct DATE
{
    u16 year;
    u8 month;
    u8 day;
}
[[
    format_read(std::format("{}-{}-{}", year, month, day)),
    transform((year * 10000) + (month * 100) + day),
    ...
]];