decor->text.str pointer must go. This removes it for conceal char,
in preparation for a larger PR which will also handle the sign case.

By actually allowing composing chars for a conceal chars, this
becomes a feature and not just a refactor, as a bonus.

Create mapping to most of the C spec and some POSIX specific functions.
This is more robust than relying files shipped with IWYU.

Remove the monolithic Decoration struct. Before this change, each extmark
could either represent just a hl_id + priority value as a inline
decoration, or it would take a pointer to this monolitic 112 byte struct
which has to be allocated.

This change separates the decorations into two pieces: DecorSignHighlight
for signs, highlights and simple set-flag decorations (like spell,
ui-watched), and DecorVirtText for virtual text and lines.

The main separation here is whether they are expected to allocate more
memory. Currently this is not really true as sign text has to be an
allocated string, but the plan is to get rid of this eventually (it can
just be an array of two schar_T:s). Further refactors are expected to
improve the representation of each decoration kind individually. The
goal of this particular PR is to get things started by cutting the
Gordian knot which was the monolithic struct Decoration.

Now, each extmark can either contain chained indicies/pointers to
these kinds of objects, or it can fit a subset of DecorSignHighlight
inline.

The point of this change is not only to make decorations smaller in
memory. In fact, the main motivation is to later allow them to grow
_larger_, but on a dynamic, on demand fashion. As a simple example, it
would be possible to augment highlights to take a list of multiple
`hl_group`:s, which then would trivially map to a chain of multiple
DecorSignHighlight entries.

One small feature improvement included with this refactor itself, is
that the restriction that extmarks cannot be removed inside a decoration
provider has been lifted. These are instead safely lifetime extended
on a "to free" list until the current iteration of screen drawing is done.

NB: flags is a mess. but DecorLevel is useless, this slightly less so