Add ext_newgrid and ext_hlstate extensions. These use predefined
highlights and line-segment based updates, for efficiency and
simplicity.. The ext_hlstate extension in addition allows semantic
identification of builtin and syntax highlights.

Reimplement the old char-based updates in the remote UI layer, for
compatibility. For the moment, this is still the default. The bulitin
TUI uses the new line-based protocol.

cmdline uses curwin cursor position when ext_cmdline is active.

MSVC has the __restrict keyword and a marco is defined for it in `win_defs.h`.

We need to disable -Warray-bounds locally for kbtree.h, but we can't
because _Pragma("GCC diagnostic pop") is broken in GCC 5.x+.

So this commit disables -Warray-bounds entirely (for GCC only).

GCC bug:
"_Pragma diagnostic 'ignored' in macro with strict-overflow not
suppressing warning fully with -Werror"
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66099

ref https://github.com/neovim/neovim/pull/7083#issuecomment-326323599
closes #7921

First stage: something compiling without klee, but with a buch of dirty
hacks - done.
Second stage: something running under klee, able to emit useful results,
but still using dirty hacks - done.

Third stage: make CMake care about clang argumnets - not done, may be
omitted if proves to be too hard. Not that klee can be run on CI in any
case.