Use Encoding::from_index instead of manually using list
Cleanup unnecessary code from Symbol
Simplify check for string containing null bytes
Move check for empty string for 1.8 mode
Use String with encoding for unmarshalling
Store encoding index with Encoding object
Store encoding indexes for Symbols
Use encoding of passed string for Symbol

This only uses the encoding if the string is not ascii compatible. In
that case the encoding for the string should be ascii.
Remove tags for passing specs
Improve specs for rb_intern3 and encodings
Fix rb_intern3 for encodings
Remove tags for passing specs