Restructure project directories (#333)

`/client/web` -> `/frontend`
`/client/cli` -> *delete for now*
`/client/native` -> *delete for now*
`/core/editor` -> `/editor`
`/core/document` -> `/graphene`
`/core/renderer` -> `/charcoal`
`/core/proc-macro` -> `/proc-macros` *(now plural)*

charcoal/src/lib.rs

@@ -0,0 +1,7 @@
+#[cfg(test)]
+mod tests {
+	#[test]
+	fn it_works() {
+		assert_eq!(2 + 2, 4);
+	}
+}