Integrate Chunk class into malloc_chunk command

[CptGibbon]

The Chunk class is complete enough for use in the malloc_chunk command, where it can simplify a little code and provide some gdb module efficiency.
The malloc_chunk command output remains the same.

[codecov-commenter]

Codecov Report

Merging #1184 (ceb735b) into dev (bddaa40) will increase coverage by 0.18%.
The diff coverage is 43.83%.

@@            Coverage Diff             @@
##              dev    #1184      +/-   ##
==========================================
+ Coverage   52.64%   52.82%   +0.18%     
==========================================
  Files         179      179              
  Lines       20102    20116      +14     
  Branches     1846     1845       -1     
==========================================
+ Hits        10582    10626      +44     
+ Misses       9117     9081      -36     
- Partials      403      409       +6     

Impacted Files	Coverage Δ
pwndbg/commands/heap.py	61.87% <28.88%> (+0.88%)	⬆️
pwndbg/heap/ptmalloc.py	40.86% <67.85%> (+3.56%)	⬆️

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