[Stream.h] make MultiTarget and findMulti public

Why MultiTarget and findMulti in Stream.h are not public members of Stream? Their implementation is by no means trivial and they seem relatively safe, so why hide this functionality from users?
I found it very useful e.g. for parsing text commands coming from serial port. I'm sure there's a lot of other cases where it may help.

Start in Stream.h by locating the declarations and implementations of MultiTarget and findMulti, then inspect how their visibility is currently defined. Done means both members are publicly accessible without changing their existing behavior; check the surrounding Stream API and available tests for compatibility.

Written by the indexing model from the issue text.