+hco various smaller cleanups and helper additions
+htp add first version of new marshalling infrastructure
!hco prepare for multipart body parser implementation

Breakingness stems from
- renaming `EnhancedString::getAsciiBytes` to `EnhancedString::asciiBytes`
- removal of `HttpForm` (for now)
- refactoring of `MultipartFormData`
=htp smaller improvements in the marshalling infrastructure
+htp add first version of new unmarshalling infrastructure, tests still missing
+htp add high-level unmarshalling API entry point, more tests
=hco,htp Upgrade to latest release-2.3-dev head
=hco Apply review feedback
=htp Add more multipart unmarshalling tests
=htp Apply remaining review feedback
Merge pull request #15554 from spray/w/un-marshalling

http: new (un)marshalling infrastructure