Add simple branch decoder (STFBranchDecoder)

Add STFBranchReader
    - Reads branch instructions from a trace, returning STFBranch
    objects
    - ~2.5x faster than STFInstReader
    - Inherits from STFBufferedReader
Refactor STFInstReader
    - Now inherits from STFBufferedReader that handles circular
    buffering, other common buffered reader tasks
    - Put filter classes in their own header
Add std::ios flag support to stf_assert
Add bit extraction utilities to util.hpp