Added integer divider, basic fp unit, and fp multiplier/divider.
in harmonica.cpp, the built-in divider is only instantiated in debug mode.
the basic fp unit implements itof, ftoi, fadd, fsub, and fneg. Some of them, like fneg, can be moved to BasicAlu later.
if there are remaining slots for functional units, the fp multplier/divider can be further split to two.

regfile.h is changed such that reg00-reg07 are declared as output ports when debug mode is off. So it's easier to monitor the reg values. It can be safely removed.