69ca487 Implement prevector::fill once (Ben Woosley)
7bad78c Drop defunct IS_TRIVIALLY_CONSTRUCTIBLE handling from prevector.h (Ben Woosley)

Pull request description:

  This is clean-up post #14651:
  * Use one implementation of `prevector::fill`, as it's possible now that the implementations are identical.
  * Only apply the `IS_TRIVIALLY_CONSTRUCTIBLE` handling to the bench file where it is used, and drop the now-unnecessary associated compat includes.

Tree-SHA512: 5930b3a17fccd39af10add40202ad97a297aebecc049af72ca920d0d55b3e4c3c30ce864c8a683355895f0196396d4ea56ba9f9637bdc7d16964cdf66c195485

d856989 warnings: Compiler warning on memset usage for non-trivial type (Lenny Maiorani)
4cb188e Drop defunct IS_TRIVIALLY_CONSTRUCTIBLE handling from prevector.h (Ben Woosley)
c412d5f Reduce redundant code of prevector and speed it up (random-zebra)
e6741d0 Add new prevector benchmarks. (random-zebra)

Pull request description:

  Backports from Bitcoin a series of optimizations for `prevector` operations, especially around resizing, which takes a significative portion of the time spent by of `ReadBlockFromDisk()`.

  Running the benchmark tool before the patch:
  ```
  #Benchmark,                    count,  min,    max,    avg,    minc,   maxc,   avgc
  -----------                    ------  ----    ----    ----    -----   -----   ----
  PrevectorClearTrivial,         3328,   312631, 329431, 320311, 812854, 856536, 832823
  PrevectorDestructorTrivial,    3328,   317011, 353344, 325141, 824243, 918711, 845382
  PrevectorResizeTrivial,        3328,   321333, 327672, 324713, 835482, 851963, 844269
  ```

  and after:
  ```
  #Benchmark,                    count, min,    max,    avg,    minc,   maxc,   avgc
  -----------                    ------ ----    ----    ----    -----   -----   ----
  PrevectorClearTrivial,         65536, 15726,  16368,  16067,  40889,  42559,  41775
  PrevectorDestructorTrivial,    65536, 15865,  16416,  16141,  41249,  42684,  41968
  PrevectorResizeTrivial,        65536, 16036,  16320,  16213,  41696,  42433,  42156
  ```

  shows that `prevector::clear()`, `prevector::~prevector()`, and `prevector::resize()` become (on avg) **20X** faster.

  Based on:
  - bitcoin#12549
  - bitcoin#14651
  - bitcoin#14715

ACKs for top commit:
  Fuzzbawls:
    utACK d856989
  furszy:
    Quite cool. utACK d856989

Tree-SHA512: 8060458bfd5373448d83a3d905fbc95379cfc1c1aed19b750c98b06a1a6a4ba85a6715865388752a5bd579676e2b2e26d3584ec5b26d51e9ba1380bc4fa73f33