trivial: make `FromColumnData` public

[agalakhov]

This makes FromColumnData trait accessible to the caller so that wrappers around get() and try_get() can be written.

Signed-off-by: Alexey Galakhov [email protected]

[Sneagan]

This is an extremely valuable change. So much so that I tried this fork. QueryRow parsing for tables with long and heterogeneous columns is a mess of explicit types and unwraps. Being able to implement a wrapper (generics, anyone? ;] ) would be a game changer.