guix: Add valgrind and test dependencies

New packages in the environment:

* valgrind (needed unless compiling with '-DSL_NO_POOL_VALGRIND').
* bash (for 'test/run-tests.sh').
* diffutils (for 'diff').
* sed

Improve 'pool_dump' function, use 'pool_item_is_free'

Replace 'printf' call with 'fprintf'; add casts, comments and intermediate
variables.

Use 'expr_list_has_only_numbers' in 'prim_equal'

Fixes a bug whenever an argument of the function (other than the first one) is
not a number.

Before this commit, the Lisp error (valid, doesn't stop the interpreter) only
happened when the first element was not a number:

  sl> (= 'a 1 1)
  Error: Unexpected non-numeric argument.

However, if it was not the first element, this is what used to happen:

  sl> (= 1 'a 1)
  src/include/expr.h:300: expr_get_generic_num: Unhandled numeric case (Symbol).

doc: Add print-str, improve description in write-to-str