`map` breaks with const-only functions and generic lambdas

[TartanLlama]

struct foo {
    void non_const() {}
};

tl::optional<foo> f;
f.map([](auto&& x) { x.non_const(); } //does not compile

This is due to the issue noted here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0826r0.html

Thanks to @markpapadakis for the report.