Naive and frame pointer stack unwinders

[bjorn3]

The naive unwinder scans the stack for words that could be function pointers, while the frame pointer unwinder uses the frame pointer linked list.

cc #60

(headcrab) exec tests/testees/hello
Starting program: tests/testees/hello
Stopped(Pid(11513), SIGTRAP)
(headcrab) _patch_breakpoint_function
(headcrab) cont
Stopped(Pid(11513), SIGTRAP)
(headcrab) bt
0000555555559265 breakpoint
000055555555927d _ZN5hello4main17hd078db076938ab99E
0000555555559316 _ZN3std2rt10lang_start28_$u7b$$u7b$closure$u7d$$u7d$17hcdd26100bac59f1bE
0000555555562ce8 _ZN3std2rt19lang_start_internal17he05790f0cb2000dfE
(headcrab) bt naive
0000555555559265 breakpoint
000055555555927d _ZN5hello4main17hd078db076938ab99E
0000555555559316 _ZN3std2rt10lang_start28_$u7b$$u7b$closure$u7d$$u7d$17hcdd26100bac59f1bE
0000555555562ce8 _ZN3std2rt19lang_start_internal17he05790f0cb2000dfE
00007ffff7fdf38f _dl_rtld_di_serinfo
0000555555559150 _start
0000555555559150 _start
0000555555559300 main
0000555555559270 _ZN5hello4main17hd078db076938ab99E
0000555555581790 __libc_csu_init
00005555555592d0 main
0000555555559150 _start
00007ffff7fe4476 _dl_rtld_di_serinfo
0000555555559150 _start
000055555555917a _start
0000555555559150 _start
(headcrab)

[nbaksalyar]

Hey, sorry for a delay with the review - this is very cool, thanks! I left a few general comments and suggestions, and this can be merged after the merge conflict is resolved.

Later we can also integrate this with rustc-demangle to have a quick win with getting real function names. I'll create a new issue for that.

[nbaksalyar]

LGTM! 👍 Merge seems to be blocked by a conflict though, could you please take a look at it?