script: bound how many operations and stack entries a script may use (#57)

v1.2.1 disabled the unbounded opcodes and capped a single pushed element at
520 bytes. What it did not add was a ceiling on how many items there can be,
so a script still pays for itself once, in bytes, and then multiplies: 520
bytes per stack entry means a script of N bytes made almost entirely of
OP_DUP occupies roughly 520N in memory, on every node that validates it.

Two limits, both from Bitcoin:

  - at most 201 operations per script, counted outside the fExec test so
    the cost of a script cannot depend on which branch a signature happens
    to take. Pushes are exempt, being already bounded by the script length
    and by MAX_SCRIPT_ELEMENT_SIZE.
  - at most 1000 entries across both stacks together, counted together
    because OP_TOALTSTACK would otherwise move the problem rather than
    solve it.

This tightens validation, so a node that does not upgrade accepts a
superset of what this one accepts and is not stranded.

Verified against the live chain: a node with these rules synced all 3910
blocks from genesis and accepted every one, so nothing in use relies on
scripts these limits would reject.

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