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/decoration.h

@@ -42,7 +42,6 @@ struct Decoration {
   // TODO(bfredl): at some point turn this into FLAGS
   bool virt_text_hide;
   bool hl_eol;
-  bool shared;  // shared decoration, don't free
   bool virt_lines_above;
   // TODO(bfredl): style, signs, etc
   DecorPriority priority;
@@ -50,7 +49,7 @@ struct Decoration {
   int virt_text_width;  // width of virt_text
 };
 #define DECORATION_INIT { KV_INITIAL_VALUE, KV_INITIAL_VALUE, 0, kVTEndOfLine, kHlModeUnknown, \
-                          false, false, false, false, DECOR_PRIORITY_BASE, 0, 0 }
+                          false, false, false, DECOR_PRIORITY_BASE, 0, 0 }
 
 typedef struct {
   int start_row;