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)*

graphene/src/lib.rs

@@ -0,0 +1,22 @@
+pub mod color;
+pub mod document;
+pub mod intersection;
+pub mod layers;
+pub mod operation;
+pub mod response;
+
+pub use intersection::Quad;
+pub use operation::Operation;
+pub use response::DocumentResponse;
+
+pub type LayerId = u64;
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum DocumentError {
+	LayerNotFound,
+	InvalidPath,
+	IndexOutOfBounds,
+	NotAFolder,
+	NonReorderableSelection,
+	NotAShape,
+}