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/helpers.rs

@@ -0,0 +1,67 @@
+use proc_macro2::{Ident, Span};
+use syn::punctuated::Punctuated;
+use syn::{Path, PathArguments, PathSegment, Token};
+
+/// Returns `Ok(Vec<T>)` if all items are `Ok(T)`, else returns a combination of every error encountered (not just the first one)
+pub fn fold_error_iter<T>(iter: impl Iterator<Item = syn::Result<T>>) -> syn::Result<Vec<T>> {
+	iter.fold(Ok(vec![]), |acc, x| match acc {
+		Ok(mut v) => x.map(|x| {
+			v.push(x);
+			v
+		}),
+		Err(mut e) => match x {
+			Ok(_) => Err(e),
+			Err(e2) => {
+				e.combine(e2);
+				Err(e)
+			}
+		},
+	})
+}
+
+/// Creates an ident at the call site
+pub fn call_site_ident<S: AsRef<str>>(s: S) -> Ident {
+	Ident::new(s.as_ref(), Span::call_site())
+}
+
+/// Creates the path `left::right` from the idents `left` and `right`
+pub fn two_segment_path(left_ident: Ident, right_ident: Ident) -> Path {
+	let mut segments: Punctuated<PathSegment, Token![::]> = Punctuated::new();
+	segments.push(PathSegment {
+		ident: left_ident,
+		arguments: PathArguments::None,
+	});
+	segments.push(PathSegment {
+		ident: right_ident,
+		arguments: PathArguments::None,
+	});
+
+	Path { leading_colon: None, segments }
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use quote::ToTokens;
+	use syn::spanned::Spanned;
+
+	#[test]
+	fn test_fold_error_iter() {
+		let res = fold_error_iter(vec![Ok(()), Ok(())].into_iter());
+		assert!(res.is_ok());
+
+		let _span = quote::quote! { "" }.span();
+		let res = fold_error_iter(vec![Ok(()), Err(syn::Error::new(_span, "err1")), Err(syn::Error::new(_span, "err2"))].into_iter());
+		assert!(res.is_err());
+		let err = res.unwrap_err();
+		let mut check_err = syn::Error::new(_span, "err1");
+		check_err.combine(syn::Error::new(_span, "err2"));
+		assert_eq!(err.to_compile_error().to_string(), check_err.to_compile_error().to_string());
+	}
+
+	#[test]
+	fn test_two_path() {
+		let _span = quote::quote! { "" }.span();
+		assert_eq!(two_segment_path(Ident::new("a", _span), Ident::new("b", _span)).to_token_stream().to_string(), "a :: b");
+	}
+}