This reverts commit b0bff57a3bc79481d89595791e3fb0e4dd10c896.

`utf_char2cells()` calls `utf_printable()` twice (sometimes indirectly,
through `vim_isprintc()`) for characters >= 128. The function can be
refactored to call to it only once.

`utf_printable()` uses binary search on ranges of unprintable characters
to determine if a given character is printable. Since there are only 9
ranges, and the first range contains only one character, binary search
can be replaced with SSE2 SIMD comparisons that check 8 ranges at a
time, and the first range is checked separately. SSE2 is enabled by
default in GCC, Clang and MSVC for x86-64.

Add 3-byte utf-8 to screenpos_spec benchmarks.

Problem:

Currently `deepcopy` hashes every single tables it copies so it can be
reused. For tables of mostly unique items that are non recursive, this
hashing is unnecessarily expensive

Solution:

Port the `noref` argument from Vimscripts `deepcopy()`.

The below benchmark demonstrates the results for two extreme cases of
tables of different sizes. One table that uses the same table lots of
times and one with all unique tables.

| test                 | `noref=false` (ms) | `noref=true` (ms) |
| -------------------- | ------------------ | ----------------- |
| unique tables (50)   | 6.59               | 2.62              |
| shared tables (50)   | 3.24               | 6.40              |
| unique tables (2000) | 23381.48           | 2884.53           |
| shared tables (2000) | 3505.54            | 14038.80          |

The results are basically the inverse of each other where `noref` is
much more performance on tables with unique fields, and `not noref` is
more performant on tables that reuse fields.

Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.

Solution:
- Enable lintlua for `test/unit/` directory.
- TODO: only `test/functional/` remains unchecked.

previous: 45fe4d11add933df76a2ea4bf52ce8904f4a778b
previous: 517f0cc634b985057da5b95cf4ad659ee456a77e

Instead of nested linked lists, store autocommands in a flat, contiguous
kvec_t, with one kvec_t per event type. Previously patterns were stored
in each node of the outer linked list, so they can be matched only once
on repeating patterns. They are now reference counted and referenced in
each autocommand, and matching is skipped if the pattern repeats. Speeds
up creation and deletion, execution is not affected.

Co-authored-by: ii14 <ii14@users.noreply.github.com>

The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.