The zlib implementation that ships with most distributions is fairly slow,
even when you allow for the zlib algorithm itself being quite slow (vs
lz4 and zstd).

There are faster implementations. `libdeflate` [1] is one such example.
According to benchmarks [2], it's maybe 2-3x faster than zlib.

This PR updates helper.cpp's compression routines to use libdeflate.

It saves ~2-3% of total execution time for me:

Creating a GB mbtiles (zlib):

```
real 2m1.706s
user 28m24.186s
sys 0m41.886s
```

Creating a GB mbtiles (libdeflate):

```
real 1m58.450s
user 27m32.579s
sys 0m51.848s
```

Snapshotting an external dependency is sorta distasteful - I worry
about how much is too much from a maintenance POV. To mitigate that,
the snapshot is a direct copy of the upstream folders, so it should
be easy to update in the future if needed.

This also lets us drop the zlib1g-devel and boost-iostreams
dependencies, which is nice.

[1]: https://github.com/ebiggers/libdeflate
[2]: zlib-ng/zlib-ng#1486

Oops: if you ever passed a non-default compression level, we'd
try to create a new compressor. But because we were using the
default constructors, we'd happily create a temporary, copy its
pointer over to the `compressor` field, then destruct the temporary,
freeing the underlying native resource, and the world would end.

Instead, delete the default constructors and require an explicit
`setLevel` call.

Also update tests to verify that the different levels work.