string->pointer and pointer->string have optional encoding arg

string->pointer and pointer->string have optional encoding arg

* test-suite/tests/foreign.test ("pointer<->string"): Add test cases.

* libguile/foreign.c (scm_string_to_pointer, scm_pointer_to_string): Add
  optional encoding, and in the pointer->string case, length arguments.

* libguile/foreign.h: Update prototypes of internal functions.
  Shouldn't affect ABI as they are internal.

* doc/ref/api-foreign.texi (Void Pointers and Byte Access): Update
  docs.