Start fixing up Matchers: namespaces, composition ops

This commit also forbids composing lvalues of composed matchers, as
per previous deprecation notice. I do not expect this to be contentious
in practice, because there was a bug in that usage for years, and
nobody complained.

docs/release-notes.md

@@ -53,6 +53,7 @@
   * The description type now must be a `const char*` or implicitly convertible to it
 * The `[!hide]` tag has been removed.
   * Use `[.]` or `[.foo]` instead.
+* Lvalues of composed matchers cannot be composed further
 
 ### Fixes
 * The `INFO` macro no longer contains superfluous semicolon (#1456)