SourceLayout: Move debugs()-related code into src/debug/ (#984)

SourceLayout: Move debugs()-related code into src/debug/ (#984)

The new library is needed to facilitate debugs() reuse and, perhaps more
importantly, reuse of other Squid code that already uses debugs() -- all
without build-breaking hacks of copying debug.cc source files and very
dangerous stub use in production running code.

The Raw class, asHex(), and other I/O manipulators are not specific to
debugs() streams. They are now in src/base/.

Also removed test-suite/debug, addressing an XXX. That test was
misplaced and not really doing anything useful -- the compiler already
tests what test-suite/debug was testing (and much more).

Also removed the no longer necessary xassert() stub from test-suite/.

TODO: Adjust the moved code to use Debug namespace. Those adjustments
are likely to trigger debug/Stream.h reorganization, simplifying exposed
debugging API. They deserve a dedicated PR.

TODO: Convert more helpers to use the new debugging library. This change
only adjusts pinger because that adjustment did not require other
significant changes.