refactor(extmarks): use a more efficient representation

marktree.c was originally constructed as a "generic" datatype,
to make the prototyping of its internal logic as simple as possible
and also as the usecases for various kinds of extmarks/decorations was not yet decided.
As a consequence of this, various extra indirections and allocations was
needed to use marktree to implement extmarks (ns/id pairs) and
decorations of different kinds (some which is just a single highlight
id, other an allocated list of virtual text/lines)

This change removes a lot of indirection, by making Marktree specialized
for the usecase. In particular, the namespace id and mark id is stored
directly, instead of the 64-bit global id particular to the Marktree
struct. This removes the two maps needed to convert between global and
per-ns ids.

Also, "small" decorations are stored inline, i.e. those who
doesn't refer to external heap memory anyway. That is highlights (with
priority+flags) are stored inline, while virtual text, which anyway
occurs a lot of heap allocations, do not. (previously a hack was used
to elide heap allocations for highlights with standard prio+flags)

TODO(bfredl): the functionaltest-lua CI version of gcc is having
severe issues with uint16_t bitfields, so splitting up compound
assignments and redundant casts are needed. Clean this up once we switch
to a working compiler version.

src/nvim/extmark_defs.h

@@ -4,23 +4,11 @@
 #include "nvim/lib/kvec.h"
 #include "nvim/types.h"
 
-typedef struct Decoration Decoration;
-
 typedef struct {
   char *text;
   int hl_id;
 } VirtTextChunk;
 
-
-typedef struct {
-  uint64_t ns_id;
-  uint64_t mark_id;
-  // TODO(bfredl): a lot of small allocations. Should probably use
-  // kvec_t(Decoration) as an arena. Alternatively, store ns_id/mark_id
-  // _inline_ in MarkTree and use the map only for decorations.
-  Decoration *decor;
-} ExtmarkItem;
-
 typedef struct undo_object ExtmarkUndoObject;
 typedef kvec_t(ExtmarkUndoObject) extmark_undo_vec_t;