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/deprecations.md

@@ -9,34 +9,6 @@ either of these is a breaking change, and thus will not happen until
 at least the next major release.
 
 
-### Composing lvalues of already composed matchers
-
-Because a significant bug in this use case has persisted for 2+ years
-without a bug report, and to simplify the implementation, code that
-composes lvalues of composed matchers will not compile. That is,
-this code will no longer work:
-
-```cpp
-            auto m1 = Contains("string");
-            auto m2 = Contains("random");
-            auto composed1 = m1 || m2;
-            auto m3 = Contains("different");
-            auto composed2 = composed1 || m3;
-            REQUIRE_THAT(foo(), !composed1);
-            REQUIRE_THAT(foo(), composed2);
-```
-
-Instead you will have to write this:
-
-```cpp
-            auto m1 = Contains("string");
-            auto m2 = Contains("random");
-            auto m3 = Contains("different");
-            REQUIRE_THAT(foo(), !(m1 || m2));
-            REQUIRE_THAT(foo(), m1 || m2 || m3);
-```
-
-
 
 ## Planned changes
 

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)