=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