RT671: export(i2s|s2i|i2v|v2i)_ASN1_(IA5|BIT)STRING

The EXT_BITSTRING and EXT_IA5STRING are defined in x509v3.h, but
the low-level functions are not public. They are useful, no need
to make them static. Note that BITSTRING already was exposed since
this RT was created, so now we just export IA5STRING functions.

Reviewed-by: Tim Hudson <tjh@openssl.org>