Character set checking for few of ASN.1 strings.

It's unclear which characters are valid in VisibleString and IA5String (based on locale etc.), but their value should be under 128, so it's worth checking.
The main motivation for writing the parser was efficiency (pyasn1 is too slow), so we should try to be careful about regression. Can you run quick microbenchmarks to see how much overhead this is? Grab a few test certs and time the decoding.