Debug Mode Lite

[Jonathan Wakely]

I think we should enable _GLIBCXX_ASSERT checks under more conditions,
e.g. if the user defines _GLIBCXX_DEBUG_LITE, to enable all low-cost
debug checks that have no ABI impact, such as range checks in vectors,
non-null checks dereferencing smart pointers.

The full _GLIBCXX_DEBUG mode alters too much and requires rebuilding
all relevant objects.

A lot of our existing _GLIBCXX_ASSERT checks are cheap anyway, the
ones that aren't should be moved to a new macro that's only enable by
the full _GLIBCXX_DEBUG macro. There are some interesting additional
checks in the Google svn branch that should be turned on by the "lite"
mode.

(Please suggest a better name, I hate the word "Lite" :-)