Proposal #3337: expose Unicode and newline translation from System.IO

[Ross Paterson]

This patch increases the need to make binary Handles a different type.

When you set the TextEncoding of an output Handle to utf16 or utf32,
does that trigger output of a BOM?

When you set the TextEncoding of an input Handle to utf16 or utf32,
does it immediately read a character looking for a BOM?

Do you really need two Newline modes per Handle?  Most Handles are
unidirectional, and even a ReadWrite Handle is reading and writing
to the same file.  The main benefit seems to be that you can apply
universalNewlineMode to any Handle, but is that worth the complication?