Update how visitors work, add NameId type

* Visitors are now defined on a per-type level, allowing for greater
  flexibility in combining and re-using behavior
* NameId is used for namespaces, which are used to index locally scoped
  variables. Syms are used for free namespaces, specifically in objects.
  All NameIDs are symbols, while not all symbols are NameIDs.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/error.rs

@@ -1,9 +1,12 @@
+use crate::syn::span::*;
 use snafu::Snafu;
 
 #[derive(Debug, Snafu)]
 pub enum Error {
-    //#[snafu(display("illegal attr key"))]
-
+    #[snafu(display("invalid assignment target"))]
+    InvalidLhs {
+        span: Span,
+    }
 }
 
 pub type Result<T, E = Error> = std::result::Result<T, E>;