Add (void*) casts to memcpy/memset invocations to appease GCC 8.

A lot of these classes are not *technically* "trivially copyable"
for one reason or another, but in all of these cases it seems
OK to me to use memcpy/memset on them. Adding a cast to void*
tells GCC that "I know what I'm doing here" and shuts up the
warning.

Debugger: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.