ref(symcache): Make parsing backwards-compatible

[loewenheim]

This puts in some indirection to make it so that old symcache versions remain parseable, even if the on-disk format changes. For the two currently supported versions, 7 and 8, this is massive overkill because they only differ in how string lengths are serialized to the string table. The intention is to enable us to add data to the format without having to throw out every old symcache immediately.

[jjbayer]

I haven't reviewed this properly, but why is there a raw:v7 but no raw:v8?

[loewenheim]

Because v7 and v8 use the same on-disk structure. I could duplicate it but they would literally be character-for-character identical. I think I need to document this better.

[jjbayer]

If the only difference between V7 and V8 is how it reads strings, I think there's a lot of potential to remove code duplication if you make SymCacheV7 generic over get_string, something like

        trait Dialect {
            fn get_string<'data>() -> &'data str;
        }

        struct V7;
        impl Dialect for V7 {
            fn get_string<'data>() -> &'data str {
                ...
            }
        }

        struct SymCache<D = V7> {
            _d: PhantomData<D>,
        }

        impl<D: Dialect> SymCache<D> {
            fn do_something(&self) {
                let s = D::get_string();
            }
        }

    type SymCacheV8 = SymCacheV7<V8>;