=str #15355: fix race with ExposedPublisher messaage in tee

 - this change makes the secondary subscription lazy
=str #15347 in `concat` propagate errors in secondary upstream properly
Merge pull request #15386 from spray/w/fix-15347

=str #15347 in `concat` propagate errors in secondary upstream properly
=str Improve `merge` signature
+par Add new `akka-parsing` module with embedded copy of shapeless 2.0.0 and parboiled 2.0.0-RC2

shapeless: https://github.com/milessabin/shapeless/tree/2b34dc0f6ef134722afc2d0e98f4515383869f43
parboiled2: https://github.com/sirthias/parboiled2/tree/e25a799ea4b6a93921a028f4be2e205d1a0215e9

The only changes made are the ones required for embedding. No strip-down yet.
+hco Add `akka-http-core` module with initial HTTP model
+hco Add low-level server and (partial) client infrastructure
Merge pull request #15356 from spray/release-2.3-dev-http

akka-http-core
=hco,str small cleanups
+hco introduce Uri.Host.inetAddresses for easy access to the actual addresses
=hco more accurate result types for HttpEntity.apply overloads
+hco introduce generic abstract accessor for HttpCredentials parameters
!hco more stringent following of plurally named container object

 - ContentDispositionType => ContentDispositionTypes
 - TransferEncoding => TransferEncodings
 - RangeUnit => RangeUnits
!hco get rid of extra HttpResponse constructor which changes ordering

The problem is that those extra constructors can easily lead to
ambiguities in overloading resolution. We should rely on named parameters
to get a similar effect than those extra constructors.
!hco #15392 use `params` instead of `parameters` consistently
!hco privatize some internal Uri.Host constructors
Merge pull request #15391 from spray/w/small-http-core-changes

+hco Small akka-http-core changes as a preparation for the Java API
Merge pull request #15367 from drewhk/wip-15355-tee-first-message-race-drewhk

=str #15355 Fix race in Tee causing a RequestMore overtaking ExposedPublisher
!str Extend StreamIO API with `idleTimeout` config setting, improve `connectTimeout` model (refs #15401)
!hco Add client-side request rendering and response parsing, refactor, strip-down

Remove everything that is out of scope for akka-http-core M1.
Things like the host- and request-level client-side API will be added back later.