Merge #12324: speed up Unserialize_impl for prevector

86b47fa741408b061ab0bda784b8678bfd7dfa88 speed up Unserialize_impl for prevector (Akio Nakamura)

Pull request description:

  The unserializer for prevector uses `resize()` for reserve the area, but it's prefer to use `reserve()` because `resize()` have overhead to call its constructor many times.

  However, `reserve()` does not change the value of `_size` (a private member of prevector).

  This PR make the logic of read from stream to callback function, and prevector handles initilizing new values with that call-back and ajust the value of `_size`.

  The changes are as follows:
  1. prevector.h
  Add a public member function named 'append'.
  This function has 2 params, number of elemenst to append and call-back function that initilizing new appended values.

  2. serialize.h
  In the following two function:
  - `Unserialize_impl(Stream& is, prevector<N, T>& v, const unsigned char&)`
  - `Unserialize_impl(Stream& is, prevector<N, T>& v, const V&)`
  Make a callback function from each original logic of reading values from stream, and call prevector's `append()`.

  3. test/prevector_tests.cpp
  Add a test for `append()`.

  ## A benchmark result is following:
  [Machine]
  MacBook Pro (macOS 10.13.3/i7 2.2GHz/mem 16GB/SSD)

  [result]
  DeserializeAndCheckBlockTest  => 22% faster
  DeserializeBlockTest => 29% faster

  [before PR]
      # Benchmark, evals, iterations, total, min, max, median
      DeserializeAndCheckBlockTest, 60, 160, 94.4901, 0.0094644, 0.0104715, 0.0098339
      DeserializeBlockTest, 60, 130, 65.0964, 0.00800362, 0.00895134, 0.00824187

  [After PR]
      # Benchmark, evals, iterations, total, min, max, median
      DeserializeAndCheckBlockTest, 60, 160, 77.1597, 0.00767013, 0.00858959, 0.00805757
      DeserializeBlockTest, 60, 130, 49.9443, 0.00613926, 0.00691187, 0.00635527

ACKs for top commit:
  laanwj:
    utACK 86b47fa741408b061ab0bda784b8678bfd7dfa88

Tree-SHA512: 62ea121ccd45a306fefc67485a1b03a853435af762607dae2426a87b15a3033d802c8556e1923727ddd1023a1837d0e5f6720c2c77b38196907e750e15fbb902

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