Merge and reorder imports

This change brings them in-line with the default settings of
rust-analyzer. Originally, I disliked this style, but by now, I've grown
used to it and prefer it slightly. Also, I like staying on the default
path since I usually don't need to check the imports to see if
rust-analyzer messed anything up (except for omitting the self:: when
importing modules defined in the current module, grr :D).

I've also come around to the idea of trailing mod definitions instead of
putting them at the top: I would also put module definitions that
contain code instead of referencing another file below the imports. It
feels weird to have code above imports. So it just makes sense to do the
same for all types of mod definitions.

If rustfmt ever gains stable support for grouping imports, I'll probably
use that instead.

.vscode/settings.json

@@ -2,7 +2,7 @@
     "files.insertFinalNewline": true,
     "rust-analyzer.cargo.features": "all",
     "rust-analyzer.imports.granularity.enforce": true,
-    "rust-analyzer.imports.granularity.group": "module",
+    "rust-analyzer.imports.granularity.group": "crate",
     "rust-analyzer.imports.group.enable": true,
     "evenBetterToml.formatter.columnWidth": 100,
 }