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This change implements the base::IsContiguousIterator type trait and
makes use of it in base::span's constructor to allow construction from
arbitrary contiguous iterators, and not just pointers.

Given the lack of std::contiguous_iterator_tag in C++14, the type trait
is especially instantiated for iterators that are known to be
contiguous. These include pointers, and the iterators for std::array,
std::string and std::vector<T> (for T != bool).

Lastly, this change adds an implementation of std::to_address, which
allows obtaining the underlying address from a pointer like type.

TBR=dcheng

Bug: 828324
Change-Id: I00d44c5edf8d86f811b7b2d32f07fa15e084d987
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2093215
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816543}