Workaround msvc #683388 in concept readable

Fix ::-targets for installed packages

add .base() to view::group_by and its iterators, fixes #1340

coerce the value written through ostream_iterator to the right type

* Implementing D1870R1.

* Making these forwarding refs.

* Making safe lowercase and removing erroneous &s.

* clang-format, nuke protected ~view_interface() destructor

* Promoting rvalues to lvalues in ranges::{r,}{begin,end}

* misc cleanup

* back out whitespace-only changes to keep this commit clean. formatting will be applied in a separate commit

* range/access.hpp doesn't need to include concepts/type_traits.hpp

In some case Boost_LIBRARY_DIRS is empty, which happens when I use a local compiled boost 1.71.0 installed to /usr/locl/lib

Instead, use modern way to link with boost libraries, so there is no need to
specify -L directly

Require prefixing range-v3:: to targets with high chance of clashing

improve compiler diagnostics by avoiding macro use in ranges::invoke

Fix warning about shadowing in adjacent_filter

use a single generic lambda instead of two to emulate concepts, where possible

a|b should not be equivalent to b(a) if the later would create a closure object

use missing template argument in concepts

more lazy range conversion constraint is more better

Apply @CaseyCarter's msvc patch to trunk instead of new-concepts-syntax

drop support for gcc <6 and clang <3.9

it should be possible to join a range of xvalue containers

Fix license version