typed arrays: re-export SizeOfArrayElementForType()

Although it is not used externally by node, it is needed by upstream and Plask.

This effectively reverts:

    commit 14448013
    Author: Aaron Jacobs <jacobsa@google.com>
    Date:   Thu Mar 15 13:26:35 2012 +1100

        typed arrays: unexport SizeOfArrayElementForType()

        It isn't used anywhere else, so made it an implementation detail in
        v8_typed_array.cc.