[DO NOT MERGE]: experiments with jemalloc and rayon

[imrn99]

Description

Scope: core / benches

Type of change: feat

Content description:

• add parallel iterators over i-cells
• add par-internals feature to core crate
  • use rayon's par_extend and collect from parallel iterators to see if we have some first touch effects
• add jemalloc feature to benches
  • see if it affects parallel performance

Additional information

• New dependency: tikv-jemallocator

Necessary follow-up

run benches with one and/or the other, along with variations on thread binding and hyperthreading.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.61345% with 79 lines in your changes missing coverage. Please review.

Project coverage is 86.82%. Comparing base (9b352cd) to head (9222a3d).

Files with missing lines	Patch %	Lines
honeycomb-core/src/cmap/dim3/basic_ops.rs	0.00%	52 Missing ⚠️
honeycomb-core/src/cmap/dim2/basic_ops.rs	0.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
- Coverage   87.41%   86.82%   -0.59%     
==========================================
  Files          72       72              
  Lines       11423    11514      +91     
==========================================
+ Hits         9985     9997      +12     
- Misses       1438     1517      +79     

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[imrn99]

the CI works again lol