Improved exactness handling for complex number parsing

Improved exactness handling for complex number parsing

When parsing non-real complex numbers, apply exactness specifiers on
per-component basis, as is done in PLT Scheme.  For complex numbers
written in rectangular form, exactness specifiers are applied to the
real and imaginary parts before calling scm_make_rectangular.  For
complex numbers written in polar form, exactness specifiers are applied
to the magnitude and angle before calling scm_make_polar.

There are two kinds of exactness specifiers: forced and implicit.  A
forced exactness specifier is a "#e" or "#i" prefix at the beginning of
the entire number, and applies to both components of a complex number.
"#e" causes each component to be made exact, and "#i" causes each
component to be made inexact.  If no forced exactness specifier is
present, then the exactness of each component is determined
independently by the presence or absence of a decimal point or hash mark
within that component.  If a decimal point or hash mark is present, the
component is made inexact, otherwise it is made exact.

After the exactness specifiers have been applied to each component, they
are passed to either scm_make_rectangular or scm_make_polar to produce
the final result.  Note that this will result in a real number if the
imaginary part, magnitude, or angle is an exact 0.

Previously, both forced and implicit exactness specifiers applied to
the number as a whole _after_ calling scm_make_rectangular or
scm_make_polar.

For example, (string->number "#i5.0+0i") now does the equivalent of:

  (make-rectangular (exact->inexact 5.0) (exact->inexact 0))

which yields 5.0+0.0i.  Previously it did the equivalent of:

  (exact->inexact (make-rectangular 5.0 0))

which yielded 5.0.

* libguile/numbers.c (mem2ureal): Receive a forced exactness specifier
  (forced_x), create and maintain our own implicit exactness specifier
  flag local to this component (implicit_x), and apply these exactness
  specifiers within this function.  Previously, we received a pointer to
  an implicit exactness specifier flag from above, and the exactness
  specifiers were applied from within scm_i_string_length.

  (mem2complex): Receive a forced exactness specifier parameter and pass
  it down to mem2ureal.  Previously, we passed down a pointer to an
  implicit exactness specifier flag instead.

  (scm_i_string_to_number): No longer create an implicit exactness
  specifier flag here, and do not apply exactness specifiers here.  All
  we do here now regarding exactness is to parse the "#e" or "#i" prefix
  (if any) and pass this information down to mem2ureal via mem2complex
  in the form of an explicit exactness specifier (forced_x).

  (scm_c_make_polar): If the cosine and sine of the angle are both NaNs
  and the magnitude is zero, return 0.0+0.0i instead of +nan.0+nan.0i.
  This case happens when the angle is not finite.

* test-suite/tests/numbers.test (string->number): Move the test cases
  for non-real complex numbers into a separate table in which the
  expected real and imaginary parts are separate entries.  Add several
  new test cases.