ENH: add storage format 2.0 with 4 byte header length size

The new format only increases the header length field to 4 bytes.
allows storing structured arrays with a large number of named columns.
The dtype serialization for these can exceed the 2 byte header length
field required by the 1.0 format.
The generic functions automatically use the 2.0 format if the to be
stored data requires it. To avoid unintentional incompatibilies a
UserWarning is emitted when this happens.
If the format is not required the more compatible 1.0 format is used.

Closes gh-4690
Merge pull request #4765 from juliantaylor/npyformat-2.0

ENH: add storage format 2.0 with 4 byte header size