feat: Add a few new memory APIs and replace old APIs

[fidgetingbits]

These are some changes that I made while porting some musl mallocng functionality, but am breaking up PRs so that it's easier to manage. So this just exposes some new API that aren't yet being called anywhere.

Hopefully are intuitively named. If people have better ideas, I'm happy to change them. If we settle on something, we can change the read_gdbvalue() calls in a separate PR later.

[codecov]

Codecov Report

Attention: Patch coverage is 18.91892% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 3.64%. Comparing base (5e77036) to head (e958747).
Report is 5 commits behind head on dev.

Files	Patch %	Lines
pwndbg/gdblib/memory.py	35.29%	8 Missing and 3 partials ⚠️
pwndbg/heap/ptmalloc.py	0.00%	6 Missing ⚠️
pwndbg/heap/structs.py	0.00%	2 Missing ⚠️
pwndbg/arguments.py	0.00%	1 Missing ⚠️
pwndbg/chain.py	0.00%	1 Missing ⚠️
pwndbg/commands/heap.py	0.00%	1 Missing ⚠️
pwndbg/commands/plist.py	0.00%	1 Missing ⚠️
pwndbg/disasm/arch.py	0.00%	1 Missing ⚠️
pwndbg/disasm/x86.py	0.00%	1 Missing ⚠️
pwndbg/enhance.py	0.00%	1 Missing ⚠️
... and 4 more

Additional details and impacted files

@@           Coverage Diff            @@
##             dev   #2155      +/-   ##
========================================
- Coverage   3.65%   3.64%   -0.01%     
========================================
  Files        196     196              
  Lines      25383   25640     +257     
  Branches    2597    2622      +25     
========================================
+ Hits         928     935       +7     
- Misses     24327   24572     +245     
- Partials     128     133       +5     

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[fidgetingbits]

With noting that there is a risk that people who have scripts that use pwndbg's gdblib could be using poi() and read_gdbvalue(). Not sure if there is an established API deprecation mechanism for changes like this, or if you want to create one, or just accept that this may break stuff for external tooling.

[gsingh93]

@fidgetingbits needs a rebase.

@disconnect3d are you good with the changes in this PR?

[fidgetingbits]

rebased now

[disconnect3d]

lgtm; @gsingh93 please merge if u are okay with those changes