`libc` doesn't have `off_t` for WebAssembly without WASI, so it falls
back to `long`.  Do the same in this crate to fix the build error when
trying to statically build libz for WebAssembly.

Rough code from `zconf.h` declaring `z_off_t`:
```c++
#ifndef Z_SOLO
#  if defined(Z_HAVE_UNISTD_H) || defined(_LARGEFILE64_SOURCE)
// [...]
#    ifndef z_off_t
#      define z_off_t off_t
#    endif
#  endif
#endif

// [...]

#ifndef z_off_t
#  define z_off_t long
#endif
```

Fixes #95.

Fix static wasm build without wasi

Upgrade ctest2, now that it no longer uses mem::zeroed

This leads to a bit of churn, but will hopefully make further changes
easier.

Per rust-lang/api-guidelines#230 .

This also avoids having something that needs to be kept updated.

This also eliminates a `cfg`.

libz-ng-sys uses the zlib-ng native API, rather than the zlib-compat
API. It exports the same Rust API (modulo the slight differences in
types), without the `zng_` prefixes, to make it easy to write Rust
software that works with both.

Add a separate Cargo.toml and README for libz-ng-sys.

Move the `build_zlib_ng` function from `build.rs` to `build_zng.rs`, and
use that as the build script for libz-ng-sys.

Add a cargo-zng script that creates the libz-ng-sys crate in a temporary
directory and runs cargo on it, to make it easy to run tests with:
```
./cargo-zng test
./cargo-zng run --manifest-path systest/Cargo.toml
```

Test libz-ng-sys in CI.

Add a libz-ng-sys crate, using the same sources with minimal duplication