This PR makes the oxipng binary process multiple files in parallel,
finally fulfilling #275. There seemed to be some debate about whether
oxipng _should_ do this or not but there's a couple of reasons I think
it makes sense:
1. The concern seemed mostly around the complexity of such a feature.
Not to worry, it was trivial* 🙂
2. Since then, oxipng has dropped from a max of something like 180
simultaneous compression trials down to 10, which is very much a good
thing but it does mean it's not utilising any more cores than that.
Some benchmarks on around 100 files on a machine with 8 cores:
Level | Master time | PR time
-|-|-
2 | 28.303 | 19.005
3 | 36.507 | 23.089
5 | 1:10.86 | 1:16.01
*Some additional changes were required in order to make sure sensible
output is printed to the terminal, since things won't be in order
anymore. Here's some example output from before:
```
Processing: tests/files/fully_optimized.png
file size = 67 bytes (0 bytes = 0.00% decrease)
File already optimized
Processing: tests/files/corrupted_header.png
Invalid PNG header detected
Processing: tests/files/verbose_mode.png
file size = 102480 bytes (12228 bytes = 10.66% decrease)
Output: tests/files/verbose_mode.png
```
And after:
```
Processing: tests/files/verbose_mode.png
Processing: tests/files/fully_optimized.png
Processing: tests/files/corrupted_header.png
tests/files/corrupted_header.png: Invalid PNG header detected
tests/files/fully_optimized.png: Could not optimize further, no change written
102480 bytes (10.66% smaller): tests/files/verbose_mode.png
```
Closes#275, #84, #169, #196 and #419.
[edit] This is the last thing I wanted to land before the next release 🥳
* Make dependency on `image` optional
After PR https://github.com/shssoichiro/oxipng/pull/481 was merged, the
`image` dependency became unused when building with debug assertions
disabled, as it is only used to implement output sanity checks when such
assertions are enabled.
The `image` crate transitively pulls a significant amount of
dependencies, so it's useful for OxiPNG users to get rid of them when
not needed.
[Cargo does not allow specifying dependencies that are only pulled when
debug assertions are
enabled](https://github.com/rust-lang/cargo/issues/7634), so the next
best way to give users some flexibility is to gate those debug
assertions behind a feature flag.
These changes add a `sanity-checks` feature flag that controls whether
the `image` crate and the related sanity checks are compiled in. This
feature is enabled by default to keep debug builds useful to catch
problems during development.
* Fix Clippy lints
* Run tests with new sanity-checks feature enabled
