Add SystemHeapAllocator to public API

The SystemHeapAllocator structure contains callbacks to be used to allocate
memory from the system heap. The allocation is to be done by default using
the functions from the C/C++ library (e.g. malloc/free).

The v8::Isolate::New() function receives a SystemHeapAllocator pointer, and
all the memory allocation from the system heap done by the newly created
Isolate (and the Isolate instance itself) will be done using the callbacks
in it.
Add internal SystemHeapAllocat{or,ed} utility classes

This SystemHeapAllocator class provides utility functions which
work with the SystemHeapAllocator defined in the public API, making
it more convenient to use.

SystemHeapAllocated is meant to be used as base class for objects
that are to be allocated using SystemHeapAllocator. It defines
operators to use placement-new with a SystemHeapAllocator, and
an allocator-aware Dispose() method to be used instead of the
global delete operator.
Implement v8::SystemHeapAllocator::{get,set}Default

Those functions access a static global which is preinitialized to an
allocator that uses the system C library malloc() and free() functions.
Add SystemHeapAllocatorObject utility class

The SystemHeapAllocatorObject class is intended to be used as a base class
for objects which are allocated via a SystemHeapAllocator. The class defines
custom new/delete operators. Allocation is done via placement-new, and the
allocator will embed a copy of itself (read: a pointer) in the allocated
memory area, so the delete operator can retrieve it to perform the
deallocation.
Base Isolate on SystemHeapAllocatorObject
Base Isolate::ThreadDataTable on SystemHeapAllocated
Base Logger on SystemHeapAllocated
Base Counters on SystemHeapAllocated
Base UnicodeCache on SystemHeapAllocated
Base DateCache on SystemHeapAllocated
Base CompilationCache on SystemHeapAllocated
Base Bootstrapper on SystemHeapAllocated
Base ThreadManager on SystemHeapAllocated
Base ContextSwitcher on SystemHeapAllocated
Base StatsTable on SystemHeapAllocated
Base OptimizingCompilerThread on SystemHeapAllocated
Base RegExpStack on SystemHeapAllocated
Base StringTracker on SystemHeapAllocated
Base KeyedLookupCache on SystemHeapAllocated
Base ContextSlotCache on SystemHeapAllocated