deprecating std.stream, std.cstream, std.socketstream

[Walter Bright]

This discussion started in the thread "Getting the const-correctness of Object 
sorted once and for all", but it deserved its own thread.

These modules suffer from the following problems:

1. poor documentation, dearth of examples & rationale

2. toHash(), toString(), etc., all need to be const pure nothrow, but it's 
turning out to be problematic for doing it for these classes

3. overlapping functionality with std.stdio

4. they should present a range interface, not a streaming one