Add Dict.map

[Minnozz]

I was migrating a library to use @rescript/core and was unable to find an alternative for Js.Dict.map in either Belt or this library. Should it be added?

[Minnozz]

For reference, the implementation of Js.Dict.map:
https://github.com/rescript-lang/rescript-compiler/blob/0bd73673276decaf5a35116a0a28a3d67887d881/jscomp/others/js_dict.res#L115-L124

[zth]

Oh yeah, that definitely sounds like a good idea. Care to PR?

[DZakh]

Should we call it mapValues? Like it's in lodash https://lodash.com/docs/4.17.15#mapValues

Also, https://lodash.com/docs/4.17.15#mapKeys can be handy as well

[Minnozz]

I added mapValues and some other bindings that I needed in #181.