Ignore the === PR STARTS HERE === commit, it's just wrong. All commits are relevant :)

This PR splits the regeneratorRuntime helper into multiple helpers, so that only the necessary ones are loaded. There are now multiple "entry points":

• regenerator, always used
• regeneratorKeys, only used when there is a yield inside a for-in
• regeneratorAsyncGen, only used for async function* () and async function () (when transpiling them through regenerator)
• regeneratorAsync, only used for async function () (when transpiling them through regenerator)

There are a bunch of new internal helpers, I added a @internal marker to not expose them as part of the public @babel/runtime API.

These are the size differences, when using inline helpers and only @babel/plugin-transform-regenerator, after giving the output to Terser:

export function* fn() {
  yield "abc";
  yield* "def";
}

export async function fn() {
  await "abc";
}

export async function* fn() {
  await "abc";
  yield "abc";
  yield* "def";
}

export function* fn() {
  for (var k in o) yield k;
}