http: Upgrade components signatues to use std::experimental::string_view

[RicoAntonioFelix]

API Changes

• All standard header fields are now placed under http::header_fields with fields names in common case
• http:extension_to_type -> http::ext_to_mime_type
• make_request(buffer_t, const size_t) -> make_request(std::string)
• Request_line::get_method -> Request_line::method
• Request_line::get_uri -> Request_line::uri
• Request_line::get_version -> Request_line::version
• Status_line::get_code -> Status_line::code
• Status_line::get_version -> Status_line::version
• Version::get_minor -> Version::minor
• Version::get_major -> Version::major

[alfreb]

@RicoAntonioFelix string view sounds like a great idea! How far away is this interface from includeos/http and Acorn integration? Would be great to make the switch.

[RicoAntonioFelix]

@alfred-bratterud I've updated the PR to outline the breaking API changes...