Revise our assertion and bug macros to work with -Wparentheses

[nmathewson]

On GCC and Clang, there's a feature to warn you about bad
conditionals like "if (a = b)", which should be "if (a == b)".
However, they don't warn you if there are extra parentheses around
"a = b".

Unfortunately, the tor_assert() macro and all of its kin have been
passing their inputs through stuff like PREDICT_UNLIKELY(expr) or
PREDICT_UNLIKELY(!(expr)), both of which expand to stuff with more
parentheses around "expr", thus suppressing these warnings.

To fix this, this patch introduces new macros that do not wrap
expr. They're only used when GCC or Clang is enabled (both define
GNUC), since they require GCC's "({statement expression})"
syntax extension. They're only used when we're building the
unit-test variant of the object files, since they suppress the
branch-prediction hints.

I've confirmed that tor_assert(), tor_assert_nonfatal(),
tor_assert_nonfatal_once(), BUG(), and IF_BUG_ONCE() all now give
compiler warnings when their argument is an assignment expression.

Fixes bug 27709.

Bugfix on 0.0.6, where we first introduced the "tor_assert()" macro.

[coveralls]

Coverage increased (+0.009%) to 49.973% when pulling 5e582c7 on nmathewson:bug27709_029 into c02f2d9 on torproject:maint-0.2.9.