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-literal.h

@@ -49,9 +49,6 @@ extern bool lit_literal_equal_type_num (lit_literal_t, ecma_number_t);
 extern bool lit_literal_equal_type (lit_literal_t, lit_literal_t);
 extern bool lit_literal_equal_charset (lit_literal_t, const lit_utf8_byte_t *, lit_utf8_size_t);
 
-extern const lit_utf8_byte_t *lit_literal_to_utf8_string (lit_literal_t, lit_utf8_byte_t *, size_t);
-extern const char *lit_literal_to_str_internal_buf (lit_literal_t);
-
 extern lit_literal_t lit_get_literal_by_cp (lit_cpointer_t);
 
 extern ecma_number_t lit_number_literal_get_number (lit_literal_t);