Backport #870 (optional project names) to 7.4.x (#871)

Unlike most of the other backports, this code couldn't be directly
translated so it had to be re-implemented. Luckily, it is very simple.
This implementation is a bit messy and heavy handed with potential
panics, but I think it's probably fine since file names that aren't
UTF-8 aren't really supported anyway. The original implementation is a
lot cleaner though.

The test snapshots are (almost) all identical between the 7.5
implementation and this one. The sole exception is with the path in the
`snapshot_middleware::project` test, since I didn't feel like adding a
`name` parameter to `snapshot_project` in this implementation.

rojo-test/serve-tests/no_name_default_project/src/third-level/default.project.json

@@ -0,0 +1,8 @@
+{
+  "tree": {
+    "$className": "IntValue",
+    "$properties": {
+      "Value": 1337
+    }
+  }
+}