Support bit-level initialization tracking.

[mheinsen]

From @mheinsen on March 15, 2013 2:31

I think that this should proceed as follows:

1. Update shadow memory tracking to be allocation-based rather than the old fragment-based system (recreated memory was updated way back in 97b8557).
2. Implement methods to retrieve/set the shadow state of memory that can be used by instrumented code and interceptors.
3. Update the instrumentation to propagate uninitialized bits through instructions.

In the long term, it would be nice to have a large block of shadow memory so that determining the location of the shadow memory for any particular byte was a simple operation (a la Memcheck, MemorySanitizer, etc.)

Copied from original issue: mheinsen#9

[mheinsen]

• Update shadow memory tracking to be allocation-based rather than the old fragment-based system (done in 6d04037)
• Implement methods to retrieve/set the shadow state of memory that can be used by instrumented code and interceptors.
• Update the instrumentation to propagate uninitialized bits through instructions.