subsume TextColumn into StringColumn

[lwhite1]

It's not clear that there's any benefit to having two columns to handle strings. the implementation of StringColumn could probably treat the different approaches to managing memory as implementation details hidden from the user

[lwhite1]

one possible implementation would be to copy both columns with different names and create a new StringColumn that delegates to the other columns. We could make the inner columns protected so they don't pollute the API.

[lwhite1]

This will be almost certainly be a breaking change.

[lwhite1]

implemented