Revert the Platform.os changes? #778

In #771 and #774, I changed Platform.os from an abstract type to a polymorphic variant.

This allows us to use the more convenient

if Platform.os === #ios {
  ...
}

instead of

if Platform.os === Platform.ios {
  ...
}

which is fine.

It also allows us to do

switch Platform.os {
  | #ios => ...
  | #android => ...
  | _ => ...
}

which is problematic as it compiles to something like

var match = ReactNative.Platform.OS;

if (match === "ios") {
  ...
} else if (match === "android") {
  ...
} else {
  ...
}

which will break Metro bundler's inlining (see a previous discussion in #239 for more details about this).

So I am not sure anymore if we should introduce a breaking change that invites people to use a problematic pattern. What do you think @MoOx?