Refactor BasicBlock, no inheritance from GenericNode

Refactor BasicBlock to not use GenericNode.

To manage BasicBlock's predecessors and successors we now use plain
std::vector.

The change also moves bunch of method definitions from header files
to implementation files.

In zlib, the change brings 3x improvement in the scheduler's memory
consumption. The --turbo-stats flag says we go 169MB -> 55MB in
the scheduler, 383MB -> 268MB overall.

BUG=

[Jarin, 2014-09-29 14:06:49]

jarin@chromium.org changed reviewers:
+ bmeurer@chromium.org

[Jarin, 2014-09-29 14:09:07]

Could you take a look, please?

[Benedikt Meurer, 2014-09-30 05:03:55]

LGTM, premature abstractification must die! :-)

[Jarin, 2014-10-06 15:10:02]

On 2014/09/30 05:03:55, Benedikt Meurer wrote:
> LGTM, premature abstractification must die! :-)

Committed as 24308.