change `lax_str` and `lax_int` to support enum values that don't inherit from `str` or `int`

[samuelcolvin]

Use the ob_type.rs logic, should be faster too.

[ThanhChinhBK]

Hi, could I work in this issue?

[lig]

@ThanhChinhBK Hello! Of cause you could. Just go for it!:)

If you'd encounter some difficulties you could submit a work in progress PR and ask for any advice you need.

Cheers!