Merge lit-unicode-*.inc.h-related generator scripts (#1767)

The `conversions` and `ranges` files had separate generators, which
shared some functionality through a 3rd python module, and were
quite similar to each other -- but also had some differences.
Moreover, as they were separate scripts, nothing forced to keep the
outputs in sync (to re-generate them at the same time from the same
inputs).

To fix these maintainability issues, this patch
- merges the two python files and part of the 3rd utility module
  into a single script while keeping the existing functionality,
- names the new file `gen-unicode.py` to align better with other
  generator script in `tools`, and
- adds some extra documentation how to retrieve the input files
  (as they are not part of the repository).

The refactoring affected the utility module as well, so this patch
also
- renames `c_source_helper.py` to `gen_c_source.py` (again, for
  naming consistency),
- reorganizes some of its functions not to export module-local
  helper functions, and
- adapts `js2c.py`, as it also uses this utility module.

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

jerry-core/lit/lit-unicode-conversions.inc.h

@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-/* This file is automatically generated by the unicode_case_conversion.py script
+/* This file is automatically generated by the gen-unicode.py script
  * from UnicodeData-9.0.0.txt and SpecialCasing-9.0.0.txt files. Do not edit! */
 
 /* Contains start points of character case ranges (these are bidirectional conversions). */

jerry-core/lit/lit-unicode-ranges.inc.h

@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-/* This file is automatically generated by the unicode_ranges.py script
+/* This file is automatically generated by the gen-unicode.py script
  * from UnicodeData-9.0.0.txt. Do not edit! */
 
 /**