Get rid of io-ts-types dependency

[sluukkonen]

As far as I can tell, io-ts-types is used only for IntFromString. This seems a bit heavyweight, since users of typera-common must also add monocle-ts and newtype-ts to their peerDependencies get rid of yarn/npm warnings.

It think it would be worth it to reimplement IntFromString from scratch to get rid of these deps. I can send a PR if you agree.

[akheron]

Sounds good, please do!