Make FlowCollector fun interface, remove redundant extensions

[qwwdfsad]

• It also saves us from copy-without-imports problem

Addresses #3037

[dkhalanskyjb]

Found several implementations of FlowCollector.

• Collect.kt. That implementation can be replaced with

public suspend inline fun <T> Flow<T>.collectIndexed(crossinline action: suspend (index: Int, value: T) -> Unit) {
    var index = 0
    collect { value ->
        action(checkIndexOverflow(index++), value)
    }
}

• Limit.kt. Doesn't simplify easily.
• In benchmarks, FlowPlaysScrabbleOpt. Simplifies easily.

These simplifications are not obviously better, so it's up to you whether to make them.

[qwwdfsad]

I've deliberately avoided touching benchmarks.
Regarding collectIndexed -- the proposed implementation is not identical, capturing mutable ints will be boxed on each emission, while spilling it into the state of flow collector explicitly leaves it as int type

[olme04]

Looks like after this change, coroutines 1.6.0 isn't backward compatible, when Flow<T>.collect(action) used in library. Is it expected?
originally reported at rsocket/rsocket-kotlin#201

[zjuhasz]

This change makes the documentation confusing in some places.

In SharedFlow.collect it says "To emit values from a shared flow into a specific collector ... collect { ... } extension should be used." but collect extension no longer exists.

In Flow.collect it says "To ensure the context preservation property, it is not recommended implementing this method directly. Instead, AbstractFlow can be used as the base type to properly ensure flow's properties." but every time a call to collect is made on any Flow we're anonymously implementing collect directly, usually with the SAM conversion.

[qwwdfsad]

@zjuhasz thanks for noticing, shared flow documentation is fixed in #3127

but every time a call to collect is made on any Flow we're anonymously implementing collect directly, usually with the SAM conversion.

Flow interface is not anonymously implemented during collect SAM-conversion, only FlowCollector is, which is perfectly safe