scm_string_tabulate, string_upcase_x, string_down_case_x,
string_titlecase_x, string_reverse_x, scm_string_tokenize): Use
size_t instead of int for indices into strings. Make sure that no
over- or underflow occurs. Thanks to Andreas Vögele!
(scm_xsubstring, scm_string_xcopy_x): Use ints for 'extended'
indices, which can also be negative.
scm_i_string_chars et al. Copious scm_remember_upto_heres have
been inserted. Made sure that no internal string pointer is used
across a SCM_TICK or a possible GC.