Add emulation specific notes capability to instr_t
This intention of this change is to allow users who do not have access to emulation client source code the ability to write instrumentation clients which can run with a binary emulation client.
This is achieved by adding an instruction note (note_t) which the emulaton client can populate and which the instrumentation client can read. Use of note_t from the instrumentation side is shown in the inscount.cpp sample.