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.

tests/SelfTest/UsageTests/Compilation.tests.cpp

@@ -79,7 +79,7 @@ namespace { namespace CompilationTests {
         REQUIRE_THROWS(throws_int(true));
         CHECK_THROWS_AS(throws_int(true), int);
         REQUIRE_NOTHROW(throws_int(false));
-        REQUIRE_THAT("aaa", Catch::EndsWith("aaa"));
+        REQUIRE_THAT("aaa", Catch::Matchers::EndsWith("aaa"));
         return true;
     }