Support type classes or implicits

NOTE: Current response by @dsyme is here: https://github.com/fsharp/fslang-suggestions/issues/243#issuecomment-916079347

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**Submitted by exercitus vir on 4/12/2014 12:00:00 AM**
**392 votes on UserVoice prior to migration**

(Updated the suggestion to "type classes or implicits", and edited it)
Please add support for type classes or implicits. Currently, it's possible to hack type classes into F# using statically resolved type parameters and operators, but it is really ugly and not easily extensible. I'd like to see something similar to an interface declaration:

``` fsharp
class Mappable =
abstract map : ('a -> 'b) -> 'm<'a> -> 'm<'b>
```

Existing types could then be made instances of a type classes by writing them as type extensions:

``` fsharp
type Seq with
class Mappable with
member map = Seq.map

type Option with
class Mappable with
member map = Option.map
```

I know that the 'class' keyword could be confusing for OO-folks but I could not come up with a better keyword for a type class but since 'class' is not used in F# anyway, this is probably less of a problem.

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/5762135)**
[Archived Uservoice Comments](../tree/master/archive/suggestion-5762135-support-for-type-classes-or-implicits.md#comments)