Use (eval-when-compile 'treesit) to save us from writing declare-function forms

[Yuan Fu]

Some of my recent changes to c-ts-mode.el uses some more C treesit functions, 
and I (again) forgot to add their declare-function forms to c-ts-mode.el, and 
Andrea has to fix it for me. The main problem for me is that (I think) you 
don’t get warning for them unless you build a non-tree-sitter build of Emacs. 
So I’m prone to forgot about them. Also it’s a burden for anyone wanting to use 
tree-sitter functions.

treesit.el already has all the declare-function forms, how about we use 
(eval-when-compile 'treesit) in other files? I believe that should alleviate us 
from adding those declare-function forms in other files?

I want to ask because IIRC I proposed this a while back but there were 
objections, the details of which I forgot.

Yuan