Update stream spec.
Markup doc.
Proofreading.
A bit of spec proofreading.
Spec: update negotiation frame layout.
Spec: update negotiation with proper types, add values description.
Change box diagrams to more readable table format.
Use more lightweight diagram for frame types.
Change wording in stream init.
Spec: Change frame types, change diagrams, add PRIORITY frame.
Spec: small update.
Add SSU link in spec.
Spec: better describe USIDs, more consistent LSIDs
Spec: split success and error return blocks.
Add READMEs to new structure of include directories.
Move includes according to the new structure.
Spec: add targets for testing.
Add decongestion strategies.
Move static_assert() to the right scope.
Spec: All LSIDs are in sender space.