Then we can just load metadata in C as a single msgpack blob. Which also
can be used directly as binarly data, instead of first unpacking all the
functions and ui_events metadata to immediately pack it again, which was
a bit of a silly walk (and one extra usecase of `msgpack_rpc_from_object`
which will get yak shaved in the next PR)

refactor(fileio): remove API shell layer encouraging unnecessary allocations

Problems:
- Illegal bytes after valid UTF-8 char cause utf_cp_*_off() to fail.
- When stream isn't NUL-terminated, utf_cp_*_off() may go over the end.
Solution: Don't go over end of the char of end of the string.

Functions like file_open_new() and file_open_fd_new() which just is a
wrapper around the real functions but with an extra xmalloc/xfree around
is an anti-pattern. If the caller really needs to allocate a
FileDescriptor as a heap object, it can do that directly.

FileDescriptor by itself is pretty much a pointer, or rather two:
the OS fd index and a pointer to a buffer. So most of the time an extra
pointer layer is just wasteful.

In the case of scriptin[curscript] in getchar.c, curscript used
to mean in practice:

N+1 open scripts           when curscript>0
zero or one open scripts   when curscript==0

Which means scriptin[0] had to be compared to NULL to disambiguate the
curscript=0 case.

Instead, use curscript==-1 to mean that are no script,
then all pointer comparisons dissappear and we can just use an array of
structs without extra pointers.

Note: this contains two _temporary_ changes which can be reverted
once the Arena vs no-Arena distinction in API wrappers has been removed.
Both nlua_push_Object and object_to_vim_take_luaref() has been changed
to take the object argument as a pointer. This is not going to be
necessary once these are only used with arena (or not at all) allocated
Objects.

The object_to_vim() variant which leaves luaref untouched might need to
stay for a little longer.

The `get_indent_str_vtab()` function currently calls `tabstop_padding()`
every time a tab is encountered (unless tabstops aren't used).
`tabstop_padding()` either does a division by 'tabstop' If 'vartabstop'
is not set, or iterates through the 'vartabstop' list to find current
tab width. 

Since the virtual column only increases, we can keep track of where the
next tabstop would be, and update this information once it was reached.

`get_indent_str_vtab()` also depends on 'listchars' "tab" value from the
current window, even though it may be called for a line from the same
buffer in a different window. In most cases, it is called with tabstops
enabled (last argument was `false`), so I split the function into one
that uses tabstops and the other that doesn't.

I removed `get_indent_str()` since I couldn't find any calls to it.

Currently having two separate memory strategies for API return values is
a bit unnecessary, and mostly a consequence of converting the hot spot
cases which needed it first. But there is really no downside to using
arena everywhere (which implies also directly using strings which are
allocated earlier or even statically, without copy).

There only restriction is we need to know the size of arrays in advance,
but this info can often be passed on from some earlier stage if it is
missing.

This collects some "small" cases. The more complex stuff will get a PR
each.

This expands on the global "don't pay for what you don't use" rules for
these special extmark decorations:

- inline virtual text, which needs to be processed in plines.c when we
  calculate the size of text on screen
- virtual lines, which are needed when calculating "filler" lines
- signs, with text and/or highlights, both of which needs to be
  processed for the entire line already at the beginning of a line.

This adds a count to each node of the marktree, for how many special
marks of each kind can be found in the subtree for this node. This makes
it possible to quickly skip over these extra checks, when working in
regions of the buffer not containing these kind of marks, instead of
before where this could just be skipped if the entire _buffer_
didn't contain such marks.