(incomplete) support for IEEE.MATH_COMPLEX

recently some record features are push into master, so time to support IEEE.MATH_COMPLEX for me.
(IEEE.MATH_COMPLEX is often used in DSP design for pre-calculated LUT etc.)
w/ following commits, i got,
```
$ make
[SNIP]
Making all in ieee
  NVC      ieee/IEEE.STD_LOGIC_1164
  NVC      ieee/IEEE.NUMERIC_BIT
  NVC      ieee/IEEE.NUMERIC_STD
  NVC      ieee/IEEE.STD_LOGIC_1164-body
  NATIVE   ieee/IEEE.STD_LOGIC_1164-body
  NVC      ieee/IEEE.NUMERIC_STD-body
  NATIVE   ieee/IEEE.NUMERIC_STD-body
  NVC      ieee/IEEE.NUMERIC_BIT-body
  NATIVE   ieee/IEEE.NUMERIC_BIT-body
  NVC      ieee/IEEE.MATH_REAL
  NVC      ieee/IEEE.MATH_REAL-body
  NATIVE   ieee/IEEE.MATH_REAL-body
  NVC      ieee/IEEE.MATH_COMPLEX
  NVC      ieee/IEEE.MATH_COMPLEX-body
** Fatal: sorry, non-constant record aggregates are not supported yet
        File ./math_complex-body.vhdl, Line 140
            return COMPLEX'(X, Y);
                           ^^^^^^
```
maybe i can workaround this missing aggregation support by editing to assign temp variable per a record element, though.