Consider evaluating `calc` before `seek_before`

Consider evaluating `calc`/`try_calc` before `seek_before`, which would enable the following setup:

```rust
#[bw(stream = st)]
struct Main {
// need to write textures offset at 0xC...

// write other fields

// ...but we don't know the offset until we're here.
// It would be nice if we could get the texture offset this way:

#[bw(try_calc = st.stream_position())]
#[br(temp)]
#[brw(seek_before = SeekFrom::Start(0xC), restore_position)]
textures_offset: u32,

// textures written here...
textures: Textures,
}
```

Currently, `try_calc` is evaluated *after* `seek_before`, so the value of `st.stream_position()` is always `0xC`.

I think it would make sense to switch the order of operations here, considering the argument to `SeekFrom` is either a constant or a field (it wouldn't make sense to use the old stream position, you'd use `SeekFrom::Current` instead), so it shouldn't break existing use cases.

Start by tracing how the `calc`/`try_calc` and `seek_before` attributes are processed for `#[bw]` and `#[brw]` fields. The change is done when `try_calc = st.stream_position()` observes the position before `seek_before`, while existing uses of constant or field-based seek offsets retain their behavior.

Written by the indexing model from the issue text.