Fix problems arising from incorrect use of various size types

E.g.,
* `ssize_t` was used where `lit_utf8_size_t` or `jerry_api_size_t`
  would have been correct,
* `lit_utf8_size_t` was used where `ecma_length_t` would have been
  correct.

Note, the patch also includes internal and public API changes:
* `ecma_string_to_utf8_string` does not return negative value if
   output buffer is not large enough to contain the string; the
   buffer is expected to be large enough. (`ecma_string_get_size`
   can be used to retrieve the required size.)
* `jerry_api_string_to_char_buffer` adapts the same logic (and
  `jerry_api_get_string_size` can be used to determine the
  required size of the buffer).

Related issue: #942

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/lit/lit-magic-strings.c

@@ -257,14 +257,14 @@ lit_compare_utf8_string_and_magic_string_ex (const lit_utf8_byte_t *string_p, /*
 extern lit_utf8_byte_t *
 lit_copy_magic_string_to_buffer (lit_magic_string_id_t id, /**< magic string id */
                                  lit_utf8_byte_t *buffer_p, /**< destination buffer */
-                                 ssize_t buffer_size) /**< size of buffer */
+                                 lit_utf8_size_t buffer_size) /**< size of buffer */
 {
   const lit_utf8_byte_t *magic_string_bytes_p = lit_get_magic_string_utf8 (id);
   lit_utf8_size_t magic_string_bytes_count = lit_get_magic_string_size (id);
 
   const lit_utf8_byte_t *str_iter_p = magic_string_bytes_p;
   lit_utf8_byte_t *buf_iter_p = buffer_p;
-  ssize_t bytes_copied = 0;
+  lit_utf8_size_t bytes_copied = 0;
 
   while (magic_string_bytes_count--)
   {