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)*

proc-macros/src/transitive_child.rs

@@ -0,0 +1,54 @@
+use crate::helper_structs::Pair;
+use proc_macro2::{Span, TokenStream};
+use syn::{Attribute, DeriveInput, Expr, Type};
+
+pub fn derive_transitive_child_impl(input_item: TokenStream) -> syn::Result<TokenStream> {
+	let input = syn::parse2::<DeriveInput>(input_item).unwrap();
+
+	let Attribute { tokens, .. } = input
+		.attrs
+		.iter()
+		.find(|a| a.path.is_ident("parent"))
+		.ok_or_else(|| syn::Error::new(Span::call_site(), format!("tried to derive TransitiveChild without a #[parent] attribute (on {})", input.ident)))?;
+
+	let parent_is_top = input.attrs.iter().any(|a| a.path.is_ident("parent_is_top"));
+
+	let Pair {
+		first: parent_type,
+		second: to_parent,
+		..
+	} = syn::parse2::<Pair<Type, Expr>>(tokens.clone())?;
+
+	let top_parent_type: Type = syn::parse_quote! { <#parent_type as TransitiveChild>::TopParent };
+
+	let input_type = &input.ident;
+
+	let trait_impl = quote::quote! {
+		impl TransitiveChild for #input_type {
+			type Parent = #parent_type;
+			type TopParent = #top_parent_type;
+		}
+	};
+
+	let from_for_parent = quote::quote! {
+		impl From<#input_type> for #parent_type {
+			fn from(x: #input_type) -> #parent_type {
+				(#to_parent)(x)
+			}
+		}
+	};
+
+	let from_for_top = quote::quote! {
+		impl From<#input_type> for #top_parent_type {
+			fn from(x: #input_type) -> #top_parent_type {
+				#top_parent_type::from((#to_parent)(x))
+			}
+		}
+	};
+
+	Ok(if parent_is_top {
+		quote::quote! { #trait_impl #from_for_parent }
+	} else {
+		quote::quote! { #trait_impl #from_for_parent #from_for_top }
+	})
+}