Add plog(x,e) opcode to FParser.

This is a resurrection of #300, adding a protected logarithm function ```plog(x,y)``` which is defined as ```x<y ? log(y)-(x-y)/y : log(x)``` to the ```FParserAD``` module. It also adds automatic differentiation support and JIT support for the new function.

In the same commit I reworked the way my intermediate bytecode representation works for the autodiff stuff, which became necessary due to the way _userfunctions_ work in FParser).

This way of implementing the new functions does not need any changes to the core FParser files.