operators for array-stores

^{Originally posted by **AlexandrHoroshih** June 4, 2021}
At the moment there is no option in basic effector api to do something like `call that event on every item from this array in store or in event payload`

Idea is to create operator which is something like this:

```ts
sampleForEach({
source: Store | Event,
clock,
target: Event | Effect // will be run for Every T from T[] in source
})
```
Live example:
https://share.effector.dev/vx6hm1il

Possible use case:

```ts
const runTransaction = createEvent()
const runTransactionGroup = createEvent()

sampleForEach({
source: runTransactionGroup,
target: runTransaction, // run all transactions from group one-by-one
})

runTransaction.watch(t => console.log('Handling transaction...', t)) // for example, we just logging all transactions

runTransaction({
type: 'transactionType',
body: 'Single',
})

runTransactionGroup([
{
type: 'transactionType',
body: 'group1',
},
{
type: 'transactionType',
body: 'group2',
},
{
type: 'transactionType',
body: 'group3',
},
])

// in logs:
Handling transaction...
{type: "transactionType", body: "Single"}

Handling transaction...
{type: "transactionType", body: "group1"}

Handling transaction...
{type: "transactionType", body: "group2"}

Handling transaction...
{type: "transactionType", body: "group3"}
```