Allow for int64 and bigints in indexed for loops

# Title of Suggestion

I propose we allow for loops of the following form:

```fsharp
// int64
for y = 0L to x do ()

// bigint
for y = 0I to x do ()
```
The existing way of approaching this problem in F# is to use the `in..do` syntax:

```fsharp
// int64
for y in 0L..x do ()

// bigint
for y in 0I..x do ()
```

Though this does make some things awkward because the indexer style may be preferred in some situations.

## Pros and Cons

The advantages of making this adjustment to F# are:

* Consistency
* No enumerator allocation and usage

The disadvantages of making this adjustment to F# are:

* It's work

## Extra information

These kinds of loops work in C#, so it's certainly possible to also allow them in F#.

Estimated cost (XS, S, M, L, XL, XXL): S-M

Related suggestions: https://github.com/fsharp/fslang-suggestions/issues/55
## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this