Fix visualize_heap

[hugsy]

Description/Motivation/Screenshots

Fixed several major bugs that prevented visualize-heap to work correctly.

Related issues/PRs:

• visualize-heap is broken #76
• heap arenas NoneType + heap-view doesn’t find heap gef#854

Depends on:

• Glibc* class now rely on ctypes.Structure gef#869

cc @0xricksanchez

How Has This Been Tested ?

"Tested" indicates that the PR works and the unit test (i.e. make test) run passes without issue.

• x86-32
• x86-64
• ARM
• AARCH64
• MIPS
• POWERPC
• SPARC
• RISC-V

Checklist

• My PR was done against the dev branch, not main.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[0xricksanchez]

Seems like something along the way of refactoring has gone missing when testing gef/heap_bugz with this branch here.
For the exact setup refer to hugsy/gef#854 (comment)

[hugsy]

Tests will fail until hugsy/gef#869 is merged.