[BREAKING] Rename forEach to forSome, add forEach matching spec

[conartist6]

This is an attempt to make Immutable more spec compliant. Currently a naive user would assume that forEach has the same implementation as the forEach function on native types, which does not involve early exit.

Also since this forEach now returns this, it can be chained!

[conartist6]

@tolmasky This is one of those changes that's very relevant to our discussion of breaking changes. I am proposing it specifically after considering what I said to you about considering the es6 API of Map and Set to be a gold standard that can be relied upon to be stable across versions. I'm going to need to do some research on whether Immutable itself has ever relied on being able to leverage this particular behavior of forEach when using forEach as an instance method of an object whose Immutable version is unknown.

[bdurrer]

That function has bitten us more than once, this is a good idea.
But I fear it will bash us again because we might miss a few cases. So I wonder if we could/should either

• have a migration-support-version of Immutable that logs warnings for every [potentially wrong] usage of a breaking feature
• Split this in a two step operation: rename it to forSome in one version and only later reintroduce forEach

[conartist6]

Two step sounds like the right thing to do to me.