[Value] new str* builtins

src/plugins/value/builtins_nonfree.ml

src/plugins/value/builtins_nonfree_string.ml

src/plugins/value/builtins_nonfree_string.mli

tests/non-free/cmp.c

tests/non-free/memchr.c

tests/non-free/memcmp.c

tests/non-free/oracle/memchr.err.oracle

tests/non-free/oracle/memchr.res.oracle

tests/non-free/oracle/strchr.err.oracle

tests/non-free/oracle/strchr.res.oracle

tests/non-free/oracle/strlen.res.oracle

tests/non-free/oracle/strnlen.res.oracle

tests/non-free/oracle/strnlen2.err.oracle

tests/non-free/oracle/strnlen2.res.oracle

tests/non-free/strchr.c

tests/non-free/strcmp.c

tests/non-free/strlen.c

tests/non-free/strnlen2.c