Improve surrogate handling readability

Improve surrogate handling readability

- add inline function to test and convert surrogates
  is_surrogate(c), is_hi_surrogate(c), is_lo_surrogate(c),
  get_hi_surrogate(c), get_lo_surrogate(c), from_surrogate(hi, lo)
- use names for BC header offsets and lengths in libregexp.c
- remove strict aliasing violations in `lre_exec_backtrack()`
- pass all context variables to XXX_CHAR macros in `lre_exec_backtrack()`