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/combined_message_attrs.rs

@@ -0,0 +1,124 @@
+use crate::helpers::call_site_ident;
+use proc_macro2::Ident;
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::parse::{Parse, ParseStream};
+use syn::Token;
+use syn::{ItemEnum, TypePath};
+
+struct MessageArgs {
+	pub _top_parent: TypePath,
+	pub _comma1: Token![,],
+	pub parent: TypePath,
+	pub _comma2: Token![,],
+	pub variant: Ident,
+}
+
+impl Parse for MessageArgs {
+	fn parse(input: ParseStream) -> syn::Result<Self> {
+		Ok(Self {
+			_top_parent: input.parse()?,
+			_comma1: input.parse()?,
+			parent: input.parse()?,
+			_comma2: input.parse()?,
+			variant: input.parse()?,
+		})
+	}
+}
+
+struct TopLevelMessageArgs {
+	pub parent: TypePath,
+	pub _comma2: Token![,],
+	pub variant: Ident,
+}
+
+impl Parse for TopLevelMessageArgs {
+	fn parse(input: ParseStream) -> syn::Result<Self> {
+		Ok(Self {
+			parent: input.parse()?,
+			_comma2: input.parse()?,
+			variant: input.parse()?,
+		})
+	}
+}
+
+pub fn combined_message_attrs_impl(attr: TokenStream, input_item: TokenStream) -> syn::Result<TokenStream> {
+	if attr.is_empty() {
+		return top_level_impl(input_item);
+	}
+
+	let mut input = syn::parse2::<ItemEnum>(input_item)?;
+
+	let (parent_is_top, parent, variant) = match syn::parse2::<MessageArgs>(attr.clone()) {
+		Ok(x) => (false, x.parent, x.variant),
+		Err(_) => {
+			let x = syn::parse2::<TopLevelMessageArgs>(attr)?;
+			(true, x.parent, x.variant)
+		}
+	};
+
+	let parent_discriminant = quote::quote! {
+		<#parent as ToDiscriminant>::Discriminant
+	};
+
+	input.attrs.push(syn::parse_quote! { #[derive(ToDiscriminant, TransitiveChild)] });
+	input.attrs.push(syn::parse_quote! { #[parent(#parent, #parent::#variant)] });
+	if parent_is_top {
+		input.attrs.push(syn::parse_quote! { #[parent_is_top] });
+	}
+	input
+		.attrs
+		.push(syn::parse_quote! { #[discriminant_attr(derive(Debug, Copy, Clone, PartialEq, Eq, Hash, AsMessage, TransitiveChild))] });
+	input
+		.attrs
+		.push(syn::parse_quote! { #[discriminant_attr(parent(#parent_discriminant, #parent_discriminant::#variant))] });
+	if parent_is_top {
+		input.attrs.push(syn::parse_quote! { #[discriminant_attr(parent_is_top)] });
+	}
+
+	for var in &mut input.variants {
+		if let Some(attr) = var.attrs.iter_mut().find(|a| a.path.is_ident("child")) {
+			let last_segment = attr.path.segments.last_mut().unwrap();
+			last_segment.ident = call_site_ident("sub_discriminant");
+			var.attrs.push(syn::parse_quote! {
+				#[discriminant_attr(child)]
+			});
+		}
+	}
+
+	Ok(input.into_token_stream())
+}
+
+fn top_level_impl(input_item: TokenStream) -> syn::Result<TokenStream> {
+	let mut input = syn::parse2::<ItemEnum>(input_item)?;
+
+	input.attrs.push(syn::parse_quote! { #[derive(ToDiscriminant)] });
+	input.attrs.push(syn::parse_quote! { #[discriminant_attr(derive(Debug, Copy, Clone, PartialEq, Eq, Hash, AsMessage))] });
+
+	for var in &mut input.variants {
+		if let Some(attr) = var.attrs.iter_mut().find(|a| a.path.is_ident("child")) {
+			let last_segment = attr.path.segments.last_mut().unwrap();
+			last_segment.ident = call_site_ident("sub_discriminant");
+			var.attrs.push(syn::parse_quote! {
+				#[discriminant_attr(child)]
+			});
+		}
+	}
+
+	let input_type = &input.ident;
+	let discriminant = call_site_ident(format!("{}Discriminant", input_type));
+
+	Ok(quote::quote! {
+		#input
+
+		impl TransitiveChild for #input_type {
+			type TopParent = Self;
+			type Parent = Self;
+		}
+
+		impl TransitiveChild for #discriminant {
+			type TopParent = Self;
+			type Parent = Self;
+		}
+	})
+}