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/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "graphite-charcoal"
+version = "0.1.0"
+authors = ["Graphite Authors <contact@graphite.design>"]
+edition = "2018"
+readme = "../README.md"
+homepage = "https://www.graphite.design"
+repository = "https://github.com/GraphiteEditor/Graphite"
+license = "Apache-2.0"
+
+[dependencies]

charcoal/src/lib.rs

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