refactor: use `eval_string', `eval_expression' etc. for consistency.

* lib/zile/zlisp.lua (evaluate_command, evaluate_string)
(evaluate_file): Rename from these...
(eval_command, eval_string, eval_file): ...to these. Adjust all
callers.
* lib/zmacs/eval.lua (evaluate_command, evaluate_expression)
(evaluate_string, evaluate_file): Rename from these...
(eval_command, eval_expression, eval_string, eval_file): ...to
these. Adjust all callers.
(loadfile, loadstring): Remove from export table.
(eval_file, eval_string): Add to export table.  Adjust all
callers.
* lib/zz/eval.lua (evaluate_command, evaluate_expression)
(evaluate_string, evaluate_file): Rename from these...
(eval_command, eval_expression, eval_string, eval_file): ...to
these. Adjust all callers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>