Enable fixed width integers by default and update documentation

[liamnichols]

• Custom mappings between schema formats and Swift Types #142
• Support custom mappings between schema formats and Swift types #114

[liamnichols]

I'm still on the fence as to wether we want this as the default behaviour or not 😅 Before merging I might double check some other generators to see how they handle fixed-width integers by default.

[LePips]

I ultimately think whatever is fine. I just have the thought that we should generate as closely to the schema as possible by default and then it's configurable to change that behavior.

[liamnichols]

So SwagGen has a similar fixedWidthIntegers flag that is false by default, and openapi-generator uses fixed width integers by default so I'm going to stick with this change and change the default behaviour 👍