I tried to fix much of the broken style in treesit.c, mainly instances
of:

  variable =
    mumble (frob (bar), (foo () + bar ()) + (baz () + quux ()))

which should actually be

  variable
    = mumble (frob (bar), (foo () + bar ()) + (baz () + quux ()))

and

  foo =
    mumble (frob (bar), 0)
      + (foo () + bar ())
      + (baz () + quux ())

which should actually be

  foo = (mumble (frob (bar), 0)
	 + (foo () + bar ())
	 + (baz () + quux ()))

I also changed many places to use FOR_EACH_TAIL and handle circular
lists in user input correctly.

Unfortunately, I don't have tree-sitter installed, so would someone
please do a smoke test on the changes before I install them?

In addition, I tried to improve the wording of the comments.  But that's
not the important part.