Move all Fornjot crates into crates/ folder

This has a few advantages:
1. There's less visual noise in the top folder, making it easier to
   become familiar.
2. It becomes more obvious, which of the folders contain Fornjot crates,
   as opposed to other crates that are not relevant outside of this
   repository.
3. It opens the door for further automation of the release process, by
   providing an easy way to detect all crates that should be published.

.github/workflows/cd.yml

@@ -137,12 +137,12 @@ jobs:
           # Publish to crates.io
           cargo run -- publish \
             --token ${{ secrets.CARGO_REGISTRY_TOKEN }} \
-            --crate ../fj-app \
-            --crate ../fj-export \
-            --crate ../fj-host \
-            --crate ../fj-interop \
-            --crate ../fj-kernel \
-            --crate ../fj-math \
-            --crate ../fj-operations \
-            --crate ../fj-viewer \
-            --crate ../fj
+            --crate ../crates/fj-app \
+            --crate ../crates/fj-export \
+            --crate ../crates/fj-host \
+            --crate ../crates/fj-interop \
+            --crate ../crates/fj-kernel \
+            --crate ../crates/fj-math \
+            --crate ../crates/fj-operations \
+            --crate ../crates/fj-viewer \
+            --crate ../crates/fj

Cargo.toml

@@ -1,15 +1,15 @@
 [workspace]
 resolver = "2"
 members = [
-    "fj",
-    "fj-app",
-    "fj-export",
-    "fj-host",
-    "fj-interop",
-    "fj-kernel",
-    "fj-math",
-    "fj-operations",
-    "fj-viewer",
+    "crates/fj",
+    "crates/fj-app",
+    "crates/fj-export",
+    "crates/fj-host",
+    "crates/fj-interop",
+    "crates/fj-kernel",
+    "crates/fj-math",
+    "crates/fj-operations",
+    "crates/fj-viewer",
 
     "models/cuboid",
     "models/group",
@@ -19,12 +19,12 @@ members = [
     "release-operator",
 ]
 default-members = [
-    "fj-app",
-    "fj-export",
-    "fj-host",
-    "fj-interop",
-    "fj-kernel",
-    "fj-math",
-    "fj-operations",
-    "fj-viewer",
+    "crates/fj-app",
+    "crates/fj-export",
+    "crates/fj-host",
+    "crates/fj-interop",
+    "crates/fj-kernel",
+    "crates/fj-math",
+    "crates/fj-operations",
+    "crates/fj-viewer",
 ]

models/cuboid/Cargo.toml

@@ -7,4 +7,4 @@ edition = "2021"
 crate-type = ["cdylib"]
 
 [dependencies.fj]
-path = "../../fj"
+path = "../../crates/fj"