Update and fix heap command

[theguy147]

Update and fix heap command

Description/Motivation/Screenshots

This PR is part of #693 and covers the following points:

• Add the missing set-arena subcommand to the heap help message.
• Fix address parsing for heap chunks command from a simple conversion to int to use parse_address() instead.
• Update heap bins fast command output in documentation.
• Fix test_cmd_heap_bins_fast test to actually look for a fastbin in the output. To do this the heap-fastbins.c had to be changed to first fill all 7 tcache bins before freeing the actual fastbin to look for.
  EDIT: heap-fastbins.c doesn't need to be udpated when disabling tcache via the environment in GDB: set environment GLIBC_TUNABLES glibc.malloc.tcache_count=0 (as pointed out to me by @irontigran )

How Has This Been Tested?

Architecture	Yes/No	Comments
x86-32	✔️
x86-64	✔️
ARM	✖️
AARCH64	✖️
MIPS	✖️
POWERPC	✖️
SPARC	✖️
RISC-V	✖️
make test	✔️

Checklist

• My PR was done against the dev branch, not master.
• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• My change adds tests as appropriate.
• I have read and agree to the CONTRIBUTING document.