Merge lexical_cast library from trunk:

* multiple optimizations and bugfixes for boost::iterator_range (refs #6786, refs #6430, refs #6663)
* documentation update
* new Unicode characters support updated
* much more tests, removed incorrect tests

[SVN r78059]

libs/conversion

@@ -1 +1 @@
-Subproject commit 1e58331bafdc716eb68f10eb78c4f3ec62a4a415
+Subproject commit eb4ad73cafed624fcb68e12b4bf11c1d195fbe62