Reason: Ship the new syntax

[chenglou]

• [x] #837
• [ ] #978
• [x] #1373
• [x] #1376
• [x] #1422
• [x] #1434
• [x] #1435
• [x] #1442
• [x] #1443
• [x] #1444
• [x] #1446
• [x] #1447
• [x] #1453
• [x] #1454
• [x] #1457
• [x] #1472
• [x] #1501
• [x] #1506
• [ ] #1507
• [x] #1508
• [x] #1510
• [x] #1533

Stretch goal:

• [x] #1494 <- edit: seems like we do have a unifying solution. Fred is working on it.
• [ ] #1514 JSX gives newcomers some of the most confusing errors

[let-def]

Working on:

• [x] #1422
• [x] #1434
• [x] #1472

[chenglou]

That was an amazingly smooth release! I've forwarded the remaining stuff to #1577.

Thanks everyone!