Don't predeclare a struct as a class (#114)

Holy moly this took a long time to debug.

MSVC gives absolutely zero warnings about if a struct/class is
predeclared as one or the other, during compilation.

However, linking also needs to happen. And as it turns out, [whether a
thing is a class or struct is part of the MSVC name mangling
spec](https://en.wikiversity.org/wiki/Visual_C%2B%2B_name_mangling).
Hence, we'd end up with e.g. this function (which references
`SymbolInfo`) as being defined in it's object file as the following
symbol:
>
`lookupSymbolsByLocation@SymbolTable@textplan@substrait@io@@qeba?AV?$vector@PEBUSymbolInfo@textplan@substrait@io@@v?$allocator@PEBUSymbolInfo@textplan@substrait@io@@@std@@@std@@AEBVLocation@234@@Z`

However, things which reference `SymbolInfo`, but which may have pulled
in the incorrect predeclaration prior, will reference it as an incorrect
symbol:
>
`lookupSymbolsByLocation@SymbolTable@textplan@substrait@io@@qeba?AV?$vector@PEBVSymbolInfo@textplan@substrait@io@@v?$allocator@PEBVSymbolInfo@textplan@substrait@io@@@std@@@std@@AEBVLocation@234@@z
...`

e.g. notice the difference between `PEBUSymbolInfo` and
`PEBVSymbolInfo`. Enqueue strange linking issues galore.

src/substrait/textplan/StructuredSymbolData.h

@@ -9,7 +9,7 @@
 
 namespace io::substrait::textplan {
 
-class SymbolInfo;
+struct SymbolInfo;
 
 // Used by the PlanRelation and Relation concepts to track connectivity.
 struct RelationData {