net: bound what a peer can make the node allocate (#73)

Three independent bounds on what an untrusted peer can make this node allocate:

- serialize.h: reject a compact size above MAX_SIZE before reading a string or a
  vector, instead of discovering it read by read. MAX_SIZE moves from main.h to
  serialize.h, where the checks live; headers_core.h includes serialize.h first,
  so every existing user still sees it.
- main.cpp: cap an orphan transaction at 10,000 inputs. Orphans are stored and
  never validated, so mapOrphanTransactionsByPrev grew with one entry per input
  of every orphan a peer cared to send.
- rpc.cpp: drop a Stratum line nested deeper than 20 before json::parse recurses
  into it. The scan tracks string state, so a worker name containing { or [ is
  no longer mistaken for nesting.

Verified: builds clean on UCRT64; the depth scanner passes a table of legitimate
Stratum lines and nesting attacks; a node built from this branch synced the chain
from genesis to 4462 with no deserialization failure and no rejected block.

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