u64 support

[jayvdb]

https://github.com/Electron100/butane/blob/9377e31/butane_core/src/codegen/mod.rs#L455-L456 says

    // Future improvement: better support unsigned integers
   // here. Sqlite has no u64, though Postgres does

https://www.postgresql.org/docs/current/datatype-numeric.html bigint is the spec compliant signed 8 bytes, which can not hold a u64 without comparisons & aggregations being incorrect.

For postgres, we could use the numeric datatype, which I guess is what that comment is referring to.

For any backend that doesnt have a data type that can hold a u64, the u64 could be stored as a string, which will compare/sort correctly as long as the value is padded, and will aggregate reasonably well. We would likely want to put that impl behind a feature flag so people dont make use of the u64-as-string trick without being aware of the trade offs.

[jayvdb]

Sample implementations for numeric datatype can be found at https://github.com/search?q=postgres+Numeric+language%3Arust&type=code . It is commonly used for storing bigdecimals. Shouldn't be too hard to add to butane.

[jayvdb]

I quickly investigated diesel, and it doesnt appear to support u64, except for MySQL.

SeaORM has a BigUnsigned which is u64. e.g. https://github.com/SeaQL/sea-orm/blob/a349f13/sea-orm-codegen/tests/expanded_with_schema_name/rust_keyword.rs#L28 However it is also only implemented in MySQL.