Work-arounds for Vivado bugs

[sgherbst]

This PR addresses several issues:

1. FIXED_TO_FLOAT is now a macro, rather than a function, due to a Vivado bug involved mixtures of signed data types and real numbers in (bug reported here).
2. FLOAT_TO_FIXED is also now a macro, due to a different Vivado bug. That bug has to due with casting real numbers to longints (reported here).
3. CALC_EXP is now a macro as well. Not strictly necessary, but since the above two functions were problematic, it seemed prudent to convert CALC_EXP to a macro as well. clog2_math is the only remaining function; it should stay a function because the macro version is very ugly (as seen in earlier versions of svreal).