imag-part of a real and non-complex number should be exact zero

For example, `(imag-part 1.0)` should evaluate to `0`, not `0.0`.

Rationale:

1. We should have `(eqv? z (make-rectangular (real-part z) (imag-part z)))`.

2. The number `1.0` is exactly on the real number line, not 
approximately on the real number line. This behavior is important for 
some numeric code near the branch cuts of functions.

3. `(eqv? 1.0 1.0+0.0i)` is #f, so the numbers should be non-`eqv?` in 
one of their parts.

I do not believe this is required by the R5RS/R7RS, but it is allowed 
and is useful.

-- Peter McGoron