move endian_fd reading into nonpure and use generic Reader

[m4b]

The idea is that in a non-pure setting, you'll typically always want endian_fd reading, which means we can use the generic Reader trait from std.

This will enable unit testing the endian fd readers by passing Cursor'd byte arrays.

We may be able to drop the no_endian_fd feature flag if this pans out the way I think it can.