Avoid intern short Strings

[gkorland]

In case the string is less than 8 bytes, will storing the string and not a pointer will save some space?

Also in some of the cases interning the IString doesn't worth it since the value is unique and in fact we just add overhead.

[Diggsey]

It's possible this could work on 64-bit platforms. But it would add a fair amount of complexity. Could maybe limit it to 7 characters which would allow the low bits to still contain the tag value for string, and the length could also be stuffed into that byte in the non-tag space.