Use LIKELY / ASSERT / ASSUME correctly

[dangelog]

The code is full of comments claiming code paths are more likely, impossible to take, and the like. In QStringIterator this showed a difference in code generation, so it'd be nice to actually have those macros in.

To avoid re-doing our compiler detection, this could / should be reimplmented in terms of Boost.Config or Qt macros. However we don't necessarily want such a dependency => ask the user to include Boost/Qt before unicode_view, detect that, and use their macros, otherwise provide default implementations of the macros that do nothing.