Homework: make PMR permeate the nlohmann JSON parser

[guoci]

From: https://youtu.be/R5PHRY5Pnlg?si=eSqaT0wj4_bGwHNi&t=949

• I used a stateless allocator method and left the nlohmann JSON library unmodified.
• This allows the std::pmr::memory_resource instance to permeate the nlohmann JSON parser, and std::pmr::polymorphic_allocator is not involved.
• There are some improvements in parsing speed over the default parse.

Benchmark                                                                        Time             CPU   Iterations UserCounters...
----------------------------------------------------------------------------------------------------------------------------------
JSON_Perf/nlohmann_JSON_Default-"citm_catalog.json"                       17851651 ns     17780746 ns           36 bytes_per_second=92.639Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc-"citm_catalog.json"           14170186 ns     14104444 ns           47 bytes_per_second=116.785Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc_devirt-"citm_catalog.json"    11097326 ns     11069338 ns           59 bytes_per_second=148.807Mi/s
JSON_Perf/nlohmann_JSON_Default-"gsoc-2018.json"                          39429436 ns     39326477 ns           21 bytes_per_second=80.7005Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc-"gsoc-2018.json"              37385009 ns     37279595 ns           22 bytes_per_second=85.1315Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc_devirt-"gsoc-2018.json"       39008590 ns     38931608 ns           18 bytes_per_second=81.519Mi/s
JSON_Perf/nlohmann_JSON_Default-"github_events.json"                       1073415 ns      1071191 ns          607 bytes_per_second=57.9866Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc-"github_events.json"            752468 ns       750393 ns         1019 bytes_per_second=82.7762Mi/s
JSON_Perf/nlohmann_JSON_MBR_stateless_alloc_devirt-"github_events.json"     724156 ns       722657 ns          954 bytes_per_second=85.9532Mi/s```