gcmh-high-cons-threshold = 16mb

Reverts 86fd6c621 which increased gcmh-high-cons-threshold from Doom's
16mb default to its upstream default (of 1gb). This was an experiment.
In theory, the user should idle enough that it would GC enough to stave
off freezes/stuttering/paging, but in practice, either users did not
idle enough or the GC didn't clean up enough when given the opportunity.
The result: terrible stuttering and freezing after long periods of use.

Back to 16mb gang.

core/core.el

@@ -300,6 +300,7 @@ config.el instead."
 ;; so we use `gcmh' to stave off the GC while we're using Emacs, and provoke it
 ;; when it's idle.
 (setq gcmh-idle-delay 5  ; default is 15s
+      gcmh-high-cons-threshold (* 16 1024 1024)  ; 16mb
       gcmh-verbose doom-debug-p)
 
 ;; Emacs "updates" its ui more often than it needs to, so slow it down slightly