This patch prepares the code for upcoming changes:

MDEV-27009 Add UCA-14.0.0 collations
MDEV-27042 UCA: Resetting contractions to ignorable does not work well

1. Adding "const" qualifiers to return type and parameters in functions:
- my_uca_contraction2_weight()
- my_wmemcmp()
- my_uca_contraction_weight()
- my_uca_scanner_contraction_find()
- my_uca_previous_context_find()
- my_uca_context_weight_find()

2. Adding a helper function my_uca_true_contraction_eq()

3. Changing the way how scanner->wbeg is set during context weight handling.
   It was previously set inside functions:
   - my_uca_scanner_contraction_find()
   - my_uca_previous_context_find()
   Now it's set inside scanner_next(), which makes the code more symmetric
   for context-free and context-dependent sequences.
   This makes then upcoming fix for MDEV-27042 simpler.