refactor: delete buffer_t

[cnjhb]

buffer_t is a string type, and glib provides a ready-made GString implementation. We don't need to maintain our own.
Our buffer_t also handles stack allocation. When stack allocation is required, I use a variable-length array as a replacement.

[codecov]

Codecov Report

❌ Patch coverage is 38.70968% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.52%. Comparing base (81b1efa) to head (de9a728).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
luaa.c	0.00%	6 Missing and 1 partial ⚠️
common/backtrace.c	0.00%	6 Missing ⚠️
common/luaobject.c	0.00%	3 Missing ⚠️
awesome.c	33.33%	2 Missing ⚠️
property.c	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4063      +/-   ##
==========================================
+ Coverage   90.47%   90.52%   +0.04%     
==========================================
  Files         940      938       -2     
  Lines       60359    60273      -86     
  Branches     1145     1132      -13     
==========================================
- Hits        54609    54560      -49     
+ Misses       5243     5217      -26     
+ Partials      507      496      -11     

Files with missing lines	Coverage Δ
ewmh.c	62.92% <100.00%> (-0.10%)	⬇️
globalconf.h	100.00% <ø> (ø)
property.c	74.00% <85.71%> (+0.11%)	⬆️
awesome.c	72.04% <33.33%> (+0.17%)	⬆️
common/luaobject.c	79.28% <0.00%> (ø)
common/backtrace.c	0.00% <0.00%> (ø)
luaa.c	68.40% <0.00%> (+0.16%)	⬆️

... and 3 files with indirect coverage changes

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[actionless]

great cleanup 🔥

[actionless]

although it now conflicts with another your PR which got merged earlier