Add InlineStdAllocator::rebind struct member

Newer versions of libc++ (as well as libstdc++) require this of
allocators. This member is needed for
std::allocator_traits<InlineStdAllocator<T, SIZE>>::rebind_alloc<U>,
which contains a typedef for InlineStdAllocator<U>. I believe this
requirement was always present, but libc++ is checking for it now.

AFAICT, the STL wouldn't be able to construct an InlineStdAllocator<U>,
because there is no default constructor, and the copy constructor
wouldn't accept a reference to InlineStdAllocator<T>. Perhaps that's OK
because this allocator is only used with std::vector, not (say)
std::list or std::map, which actually need to allocate internal node
types.

std::allocator_traits::rebind_alloc can automatically handle custom
allocators with type parameters, but not with non-type parameters
(i.e. It can't handle "size_t SIZE = 4"). See
https://stackoverflow.com/a/34863387.

Bug: b/175635923
Test: treehugger
Change-Id: I0cceb47ab780414202cef1ed54fa7deb3773d01a