Merge {marshal,unmarshal}TableValue into marshalValue.

[petermattis]

This allows easy setting of "typed" values with the standard key/value
methods which will be used shortly for setting and retrieving the
"descriptor ID" value for namespace keys.

[tbg]

seems awkwardish to panic when you could return an error.

[petermattis]

An error might be ignored or logged. I wanted something more forceful to as a reminder to implement this support. Should be trivial to do, just haven't done it yet.

[tbg]

LGTM. It's a bit hard to embrace the copious amounts of reflection since we've always tried to avoid it wherever possible, but this being a client library, us having decided to go down this road, and the simplification in this case being significant, this is certainly the right thing to do.

[petermattis]

FWIW, the database/sql package has similar amounts of reflection internally in order to deal with type conversions in the {Row,Rows}.Scan() method. The other bits of reflection to walk over the fields of structs is pretty much required for a Go ORM. The reflection certainly complicates the implementation, but I think it leaves the API usable. I'm very open to alternatives that are both reasonably clean and similarly performant.

[tbg]

No need to convince me, I'm on your side.