Add optional length parameter to GenericStringStream constructor

[masseman]

In order to be able to reference sub-strings (segments that are not zero-terminated) in GenericStringStream, a length parameter is necessary.

If the length parameter is not used, the compiler will optimize away the comparisons added in Peek() and Tell(), so this modification should not have any impact on performance.

[coveralls]

Coverage remained the same at 99.938% when pulling bed701a on masseman:master into fbcd7ac on miloyip:master.

[stervo4ka]

If the length parameter is not used, the compiler will optimize away the comparisons added

That's not true in general. Compiler might optimize the comparisons in simple cases, but consider this option:

void f(StringStream &stream) {
    // Do something.
}

In this case compiler cannot know whether the length is used.
Here is a proof https://godbolt.org/g/qMlFyH

[miloyip]

There is already a MemoryStream which can be used, though lack of documentation.

[masseman]

@miloyip Thanks, I will use that instead!
@stervo4ka Good point.