Fast constructor for constructing a larger ShortString from a shorter one

[oxinabox]

I think there should be a fast way to convert one short string into another, larger one.
This would be useful for implementing cmp and for making promotion work

[xiaodaigh]

This already works after #25 and your PR earlier right?

Re-open if I got this wrong

ss3"abc" == ss7"abc" # true

[oxinabox]

Oh yeah I forgot I opened an issue about it.