Generic Complex type (#9590)

* remove `**`
* const `im` can now be used with Complex64
* converters from float|int to Complex are replaced by procs
* converters between various Complex types must stay to allow usage
of `im` with Complex64
* limit types for `+`, `-`, `/`, and `*` between Complex and float
* add `pow` for Complex and a number
* complex type changes
* unpublish approximation function

changelog.md

@@ -55,7 +55,10 @@ proc enumToString*(enums: openArray[enum]): string =
   slightly. The `dumpLisp` macro in this module now outputs an
   indented proper Lisp, devoid of commas.
 
-- In `strutils` empty strings now no longer matched as substrings anymore.
+- In `strutils` empty strings now no longer matched as substrings
+  anymore.
+
+- Complex type is now generic and not a tuple anymore.
 
 ### Language additions