Consistent type for 'readMaybe' and 'readEither'

[chshersh]

The two functions are used to parse types with the Read instance from String. However, they have a slightly different type (one is more polymorphic than the other):

• readMaybe :: Read a => String -> Maybe a
• readEither :: (ToString a, Read b) => a -> Either Text b

I think this difference in behaviour can be surprising for some users. So I propose to chose one of the following options:

1. Both work with monomorphic String
2. Both work with polymorphic ToString

Both approaches have benefits and drawbacks: String is more beginner-friendly and less surprises for relude users, ToString is more flexible and convenient.

My initial thought is that both functions should work with String, and we can have a module like Relude.Extra.Parse/Read/String where we can put more polymorphic functions, including the ones proposed in #161.

[vrom911]

Nice catch! I also give my vote to the first option. It is less surprising and breaking.