In a72c1c1a (golang_str: bstr/ustr iteration) things were initially
implemented to follow Go semantic exactly with bytestring iteration
yielding unicode characters as explained in
https://blog.golang.org/strings. However this makes bstr not a 100%
drop-in compatible replacement for std str under py2, and even though my
initial testing was saying this change does not affect programs in
practice it turned out to be not the case.

For example with bstr.__iter__ yielding unicode characters running
gpython on py2 with builtin str patched to be bstr will break sometimes
when importing uuid:

There uuid reads 16 bytes from /dev/random and then wants to iterate
those 16 bytes as single bytes and then expects that the length
of the resulting sequence is exactly 16:

     int = long(('%02x'*16) % tuple(map(ord, bytes)), 16)

     ( https://github.com/python/cpython/blob/2.7-0-g8d21aa21f2c/Lib/uuid.py#L147 )

which breaks if some of the read bytes are higher than 0x7f.

Even though this particular problem could be worked-around with
patching uuid, there is no evidence that there will be no similar
problems later, which could be many.

-> So adjust bstr semantic instead to follow semantic of str under py2
   and introduce uiter() primitive to still be able to iterate
   bytestrings as unicode characters.

This makes bstr, hopefully, to be fully compatible with str on py2 while
still providing reasonably good approach for strings processing the
Go-way when needed.

Add biter as well for symmetry.

See

    nexedi/pygolang!21 (comment 170754)
    nexedi/pygolang!21 (comment 170782)
    ...

and

    nexedi/pygolang!21 (comment 206044)

for discussion on iter(bstr) topic.