ENH: make OSstream indentation adjustable

- this is principally for cases where reduced indentation is desired,
  such as when streaming to a memory location. If the indentation size
  is zero or one, only a single space will be used to separate the
  key/value.

  This change does not affect the stream allocation size, since the
  extra data falls within the padding.

ENH: relocate label/scalar sizes from Istream to IOstream.

- could allow future use for output streams as well?

  Due to padding, reorganization has no effect on allocated size
  of output streams.

STYLE: add read/write name qualifier to beginRaw, endRaw

- removes ambiguity for bi-directional streams

STYLE: fix inconsistent 'const' qualifier on std::streamsize

- base Ostream was without const, some derived streams with const