Losslessly optimize PNG files

Losslessly optimizes all of the PNG files in the repo. Done with:

```
oxipng -o max -a -s
oxipng -o max --zopfli -a -s
```
diff --git a/src/img/coverage-branch-counting-01.png b/src/img/coverage-branch-counting-01.png
index c445f35..7c6c845 100644
--- a/src/img/coverage-branch-counting-01.png
+++ b/src/img/coverage-branch-counting-01.png
Binary files differ
diff --git a/src/img/dataflow-graphviz-example.png b/src/img/dataflow-graphviz-example.png
index 718411a..7baa37e 100644
--- a/src/img/dataflow-graphviz-example.png
+++ b/src/img/dataflow-graphviz-example.png
Binary files differ
diff --git a/src/img/github-cli.png b/src/img/github-cli.png
index c3b0e77..88ba95f 100644
--- a/src/img/github-cli.png
+++ b/src/img/github-cli.png
Binary files differ
diff --git a/src/img/github-whitespace-changes.png b/src/img/github-whitespace-changes.png
index 9a19a10..e235a30 100644
--- a/src/img/github-whitespace-changes.png
+++ b/src/img/github-whitespace-changes.png
Binary files differ
diff --git a/src/img/llvm-cov-show-01.png b/src/img/llvm-cov-show-01.png
index 35f0459..ce4dec1 100644
--- a/src/img/llvm-cov-show-01.png
+++ b/src/img/llvm-cov-show-01.png
Binary files differ
diff --git a/src/img/other-peoples-commits.png b/src/img/other-peoples-commits.png
index e4fc2c7..0c949d8 100644
--- a/src/img/other-peoples-commits.png
+++ b/src/img/other-peoples-commits.png
Binary files differ
diff --git a/src/img/rustbot-submodules.png b/src/img/rustbot-submodules.png
index c2e6937..c099fdf 100644
--- a/src/img/rustbot-submodules.png
+++ b/src/img/rustbot-submodules.png
Binary files differ
diff --git a/src/img/submodule-conflicts.png b/src/img/submodule-conflicts.png
index e90a6bb..5d4caf0 100644
--- a/src/img/submodule-conflicts.png
+++ b/src/img/submodule-conflicts.png
Binary files differ
diff --git a/src/img/wpa-initial-memory.png b/src/img/wpa-initial-memory.png
index b602066..177d92c 100644
--- a/src/img/wpa-initial-memory.png
+++ b/src/img/wpa-initial-memory.png
Binary files differ
diff --git a/src/img/wpa-stack.png b/src/img/wpa-stack.png
index 29eb5a5..a4a7135 100644
--- a/src/img/wpa-stack.png
+++ b/src/img/wpa-stack.png
Binary files differ
diff --git a/src/queries/example-0.png b/src/queries/example-0.png
index 14b46c4..dd67d5f 100644
--- a/src/queries/example-0.png
+++ b/src/queries/example-0.png
Binary files differ