Faster conversion

[AliceRixte]

First of all, thank you for this work ! 🥳

I would like to move convert inside the Convertible class. Indeed, when a conversion cannot fail, using the convert is needlessly slowed down by pattern matching on ConvertResult.

Moving convert inside the Convertible class would allow to define it directly, and therefore making it more efficient.

I opened a PR #18 for this.

Cheers !