[lmi] PATCH: Use C++17 standard library if available

[Vadim Zeitlin]

 Hello again,

 I'd like to submit a simple PR

        https://github.com/vadz/lmi/pull/71

which doesn't change anything for g++6 but fixes the build with MSVS (which
defines LMI_COMPILER_HAS_CXX17_STDLIB in config_msvc.hpp that I have in my
local branch) and streamlines things a bit for g++7 and clang6, both of
which have std::any and std::optional support.

 I'm still not sure if it's really a good idea to use "Exp" namespace here,
but this could always be changed later, even if this PR is applied.

 Thanks in advance,
VZ