Guard u128 and i128 behind optional integer128 feature

[juntyr]

This PR adds an optional integer128 feature that guards the usage of i128 and u128. Therefore, any library using serde with either of these types should now enable the feature. This PR is heavily inspired by #304.

• I've included my change in CHANGELOG.md

[juntyr]

@torkleyy @kvark @caiiiycuk I've started the implementation to add an integer128 feature. So far the functionality should be there, but some aspects still have to be refactored (e.g. the serialize dispatch code duplication). What do you think of the general design? How should the CI be adapted? I know we can check if compilation for wasm succeeds, but should e.g. the tests be run as well?

[kvark]

Thank you!
I believe the direction is sound

[torkleyy]

Looks good to me!