fix: add better struct pick type

[thomasvargiu]

Add better Pick type to Struct and ReadonlyStruct modules.

Example:

import { pick } from 'fp-ts-std/Struct'

const AttributesKeys = ['key1' as const, 'key2' as const]
const getAttributes = <A extends Record<string, unknown>>(x: A) =>
    pipe(x, pick(AttributesKeys))

declare const y: { key1: string, foo: string }

const result = getAttributes(y) 

Currently:

typeof result = {
    key1: string
    key2: unknown
}

With this MR:

typeof result = {
    key1: string
}

[thomasvargiu]

Honestly I'm not sure. It could be too much strict as a type

[thomasvargiu]

Definitely it will cause other problems