Adding Schema.evolve Function

const s = Schema.Struct({
    a: Schema.String,
    b: Schema.Number
});

// this works but it be could nicer
const newSchema = Schema.Struct( 
    Struct.evolve(s.fields, {
        a: Schema.compose(Schema.NumberFromString)
    })
);

// this is nicer
const newSchema = Schema.evolve(s, { a: Schema.compose(Schema.NumberFromString) })