On (my) linux, perftest reports:
- ~40% gain for FileReadStream (Take() loop),
- ~10% gain for ReaderParse_DummyHandler_FileReadStream.

With the same logic applied to BasicIStreamWrapper, which thus can now
also be created with a user buffer, performances align with those of
FileReadStream (same buffer size).

The "unbuffered" versions (added for FileReadStream) work solely with
the internal peekBuffer (Ch[4]) and are measured in perftest.  When
performances don't matter much, they can avoid the use of large
stack/heap buffers.