adds scstr_t to + fixes parenthesis bug in sstrsplit_a macro scstr_t-final

use 'convert' as more precise term than 'cast' for the conversion from sstr_t to scstr_t

unifies naming of sstr_t related and scstr_t related functions

adds remaining documentation for the scstr functions

overflow of sstrnlen should return SIZE_MAX instead of zero

adjusts documentation of UCX string types, converters, and constructors

finally removes the underscore of ugliness from ucx_str_cmp() and ucx_str_casecmp()

renames int and longint distance and compare functions according to the new scheme

renames ucx_doublecmp() and ucx_floatcmp() to ucx_cmp_double() and ucx_cmp_float()

renames ucx_ptrcmp() to ucx_cmp_ptr()

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