dict2 is not tail recursive

I got the following error when decoding a big data structure:
> RangeError: Maximum call stack size exceeded

I'm not sure if this repo is actively maintained so I won't bother creating a PR, but here is a tail recursive version of `dict2` if anyone needs it:

```elm
dict2 : Decoder comparable -> Decoder v -> Decoder (Dict comparable v)
dict2 keyDecoder valueDecoder =
Decode.keyValuePairs valueDecoder
|> Decode.andThen (decodeDictFromTuples keyDecoder)

decodeDictFromTuples : Decoder comparable -> List ( String, v ) -> Decoder (Dict comparable v)
decodeDictFromTuples keyDecoder tuples =
let
helper : List ( String, v ) -> Dict comparable v -> Decoder (Dict comparable v)
helper remainingTuples dictAcc =
case remainingTuples of
[] ->
Decode.succeed dictAcc

( strKey, value ) :: rest ->
case Decode.decodeString keyDecoder strKey of
Ok key ->
helper rest (Dict.insert key value dictAcc)

Err error ->
Decode.fail (Decode.errorToString error)
in
helper tuples Dict.empty
```

Start by locating the existing `dict2` implementation and compare it with the tail-recursive version in this issue. Exercise it with a large JSON data structure to reproduce the stack overflow, then confirm decoding completes without the RangeError and existing behavior remains intact.

Written by the indexing model from the issue text.