Value representation overhaul

[graydon]

There are a few things to do in broad overhaul of the fundamental ScVal / RawVal types. This might take several separate PRs / be done in a sort of "disruption-minimzing" change order, but we need to do I think most of the following:

Tasklist

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1. Remove BitSet from ScVal / RawVal -- nobody's using it
2. null out dead objects in object table rather than rolling back on failed subcontract calls (will set read-tickets to unreadable in read tickets #132
3. add small-value cases to all number, symbol, bytes and slice rawvals
4. add {iu}256
5. Object subtypes #583
   enhancement +1
   
6. Constant slices #584
   enhancement +1
   
7. Consider alternative forms for Symbol #463
   +0
8. Replace ScObject with ScOption. Unify everything under ScVal. stellar-xdr#64
   +0
9. Log / diagnostics from guest memory static strings #229
   +0
   
10. Type: Symbol, etc: Cannot be used in match statements #198 postponed
11. A bunch of simple renamings #689 postponed
12. Value representation overhaul stellar-xdr#70
    +0
13. Value representation overhaul rs-stellar-xdr#234
    +0
14. Value and object representation overhaul #682
    +0
15. Value and object representation overhaul rs-soroban-sdk#879
    +0
16. Preliminary value overhaul change stellar-core#3673
    +0

[graydon]

Everything on this list is either done or postponed to a followup.