Fix string.base_string_to_int succeeding incorrectly.

string.base_string_to_int would incorrectly succeed on an input string
containing a code point of multiple code units.  string.foldl_between
expects the length of the string in code units, but was passed the
length of the string in code points.

library/string.m:
	As above.

tests/general/base_string_to_int_test.exp:
tests/general/base_string_to_int_test.m:
	Extend test case.