Read complex numbers where both parts are inexact decimals

Read complex numbers where both parts are inexact decimals

Thanks to Bill Schottstaedt for reporting this problem!

* libguile/numbers.c (mem2ureal): Don't be misled by *p_exactness
  being INEXACT on entry (as is possible when reading a complex
  number): use local exactness variable x which starts as EXACT.
  Call mem2decimal_from_point () with &x instead of p_exactness.

* test-suite/tests/numbers.test ("string->number"): Add complex number
  tests suggested by Bill.