Do not use using namespace std;

[omoerbeek]

it causes ambiguity if both std::string_view and boost::string_view are in scope
As seen on OpenBSD (sorry to have missed that earlier):

doh.cc:658:85: error: reference to 'string_view' is ambiguous
static bool getHTTPHeaderValue(const h2o_req_t* req, const std::string& headerName, string_view& value)
                                                                                    ^
./views.hh:31:14: note: candidate found by name lookup is 'string_view'
using boost::string_view;
             ^
/usr/include/c++/v1/string_view:771:37: note: candidate found by name lookup is 'std::__1::string_view'
typedef basic_string_view<char>     string_view;
                                    ^
doh.cc:662:3: error: reference to 'string_view' is ambiguous

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