oximeter could support more integer types

[bnaecker]

When we originally wrote oximeter, we supported the bare minimum of data types to streamline prototyping. We should really support more integer types. This should be a pretty mechanical change that adds, for each type:

• A new oximeter::FieldType variant
• A new oximeter::DatumType variant
• A new oximeter::Datum variant
• Possibly cumulative types for these, although that is probably only reasonable for the widest types
• Possibly implement histogram support for these new integer types
• New model types, oximeter_db::model::Db{Datum,Field}Type and field / measurement row types

The most important type to actually support is u64. We're currently limited to i64, which is really huge but technically not big enough to support some values (e.g., the kstats for bytes received on a datalink are technically a u64). But given how mechanical this all should be, I'd also argue for adding pretty much all the integral types.

[bnaecker]

Closed by #4091