Boost\libs\coroutine2 failed to build due to error C2668 on MSVC.

[QuellaZhang]

Issue description:
Boost\libs\coroutine2 failed to build due to error C2668 on MSVC. Could you please take a look? We ues boostorg/boost@ea9f9fb from Boost master branch. from Boost master branch.

Build step:

1. git clone -c core.autocrlf=true --recursive ​https://github.com/boostorg/boost.git boost
2. open a VS 2019 x64 command prompt and browse to boost
3. .\bootstrap
4. .\b2 headers variant=release --build-dir=..\out\x64rel address-model=64
5. .\b2 variant=release --build-dir=..\out\x64rel address-model=64
6. .\b2 -j4 variant=release --build-dir=..\out\x64rel libs\coroutine2\test

Error info:
test_coroutine.cpp
libs\coroutine2\test\test_coroutine.cpp(538): error C2668: 'std::begin': ambiguous call to overloaded function
.\boost/coroutine2/detail/pull_coroutine.hpp(301): note: could be 'boost::coroutines2::detail::pull_coroutine::iterator std::begin(boost::coroutines2::detail::pull_coroutine &)' [found using argument-dependent lookup]
with
[
T=int
]
.\boost/coroutine2/detail/pull_coroutine.hpp(285): note: or 'boost::coroutines2::detail::pull_coroutine::iterator boost::coroutines2::detail::begin(boost::coroutines2::detail::pull_coroutine &)' [found using argument-dependent lookup]
with
[
T=int
]
libs\coroutine2\test\test_coroutine.cpp(539): note: while trying to match the argument list '(boost::coroutines2::detail::pull_coroutine)'
with
[
T=int
]
libs\coroutine2\test\test_coroutine.cpp(538): error C2668: 'std::end': ambiguous call to overloaded function
.\boost/coroutine2/detail/pull_coroutine.hpp(307): note: could be 'boost::coroutines2::detail::pull_coroutine::iterator std::end(boost::coroutines2::detail::pull_coroutine &)' [found using argument-dependent lookup]
with
[
T=int
]

[jwakely]

This is a dup of #42