better help text for "match -> if let" lint

Implements the suggestion from #87.

Changes span_help_and_lint(), which is only used for this lint,
to use fileline_help() instead of span_help() to avoid printing
the span twice.

Also adds complete suggested new code. I had to distinguish
between blocks, which need no additionals braces, and other exprs.

src/utils.rs

@@ -77,10 +77,10 @@ pub fn span_lint(cx: &Context, lint: &'static Lint, sp: Span, msg: &str) {
     cx.span_lint(lint, sp, msg);
 }
 
-pub fn span_help_and_lint(cx: &Context, lint: &'static Lint, span: Span, 
+pub fn span_help_and_lint(cx: &Context, lint: &'static Lint, span: Span,
         msg: &str, help: &str) {
     span_lint(cx, lint, span, msg);
     if cx.current_level(lint) != Level::Allow {
-        cx.sess().span_help(span, help);
+        cx.sess().fileline_help(span, help);
     }
 }