Refactor xref management

[Rot127]

xrefs are currently stored in RzAnalysis as hash tables.

HtUP *ht_xrefs_from;
HtUP *ht_xrefs_to;

Depending on the number of xrefs and on the way of access this is very slow.

For example:

• Iterator over all call xrefs requires iteration over all xrefs: O(n)
• Get all xrefs from/to a certain address range. As above it requires O(n).
  • Assume for example we want to iterate over all basic blocks and for each BB the xrefs. This is then O(|BBs| * |n|). Not ok.

Because the xrefs are such a vital information source it is worth to refactor it I think.
Especially adding new API functions to allow iteration over a specific subset of xrefs.

I think it is the simplest to add indices for different xref types and requests.