allow single_view's data to be mutable

add an append_view class


[SVN r27878]

libs/spirit

@@ -1 +1 @@
-Subproject commit ae1d48367c43ee73f844034d71c56c165b28d253
+Subproject commit 93f6f8bcfd46a14e91b23c5de3f1948c17aa32e1