syntax::diagnostic: Remove unnecessary traits

This removes trait `handler` and `span_handler`,
and renames `HandlerT` to `Handler`, `CodemapT` to `SpanHandler`.

src/libsyntax/ext/base.rs

@@ -12,7 +12,6 @@ use ast;
 use ast::Name;
 use codemap;
 use codemap::{CodeMap, Span, ExpnInfo};
-use diagnostic::span_handler;
 use ext;
 use ext::expand;
 use parse;