Use different union syntax

[JSAbrahams]

Current Issue

Typing syntax for union looks a bit like sets, might be a bit confusing.

High-level description of the feature

Use same union syntax as Python, Typescrpt (other?) instead of current, so:
def x: {A, B, C ...} := ... changes to def x: A | B | C := ....

This also has the added benefit of being more familiar to uses of those languages.
Plus, it just looks better, now we don't mistake it for a set accidentally.

Description of potential implementation

• Update parser to deal with new syntax.
• Update docs (BNF) to reflect change.
• Also change Python output to use this syntax as per PEP 604.