Feature/visitor

[janstarke]

This PR provides a generic access to the parsed evtx structure, without a need to parse json (which would make everything very slow)

Usage example:

struct MySpecialEvtxDataStructure {}
struct MyVisitor {}
impl EvtxStructureVisitor for MyVisitor {
  type VisitorResult = Option<MySpecialEvtxDataStructure>;
  /* ... */
}
/* ... */
let records = parser.records_to_visitor(|| MyVisitor::new());

I used evtxgrep as PoC to see if everything works as desired, keep as much changes as possible outside of evtx

I also use this in https://github.com/janstarke/evtx2bodyfile

[janstarke]

Another PoC is https://github.com/janstarke/evtx2bodyfile, which I use regularly in forensic investigations