Merge lexical_cast and some conversion docs and test from trunk:

* fixed int128 and uint128 conversions (fixes #8790)
* fixed warnings from Boost Inspection Report

[SVN r85160]

test/implicit_cast_fail.cpp

@@ -19,6 +19,8 @@ struct foo
 int test_main(int, char*[])
 {
     foo x = implicit_cast<foo>("foobar");
-    (void)x;  // warning suppression.
+    (void)x;            // warning suppression.
+    BOOST_CHECK(false); // suppressing warning about 'boost::unit_test::{anonymous}::unit_test_log' defined but not used
     return 0;
 }
+