Using from_u8/match_u8 with data pulled from streams

[katyo]

The case, when file data pulled from stream, requires little different API because we don't sure that we have enough data.

I think from_u8() and match_u8() should return Option<_> where None means "not enough data to determine".

[katyo]

As alternative solution I would like to have a way to know how much data actually required to match or detect type.