type inference for container literals (list, dict, set) and comprehensions

[carljm]

We should start with a pretty dumb version of this (infer solely based on the types present at construction, maybe unioned with Unknown), which will be quick and easy to implement and be fine for a lot of cases.

We have a lot of design decisions to make about how we want to improve from there. It likely intersects with #136 and bidirectional checking.

[Andrej730]

Just came across this too with simple constructors like so.

y = [1,2,3]
# list[Unknown]
reveal_type(y)

And some comprehensions cases:

a: list[int] = [1,2,3]
b = [i for i in a]
# Revealed type: `@Todo`
reveal_type(b)

d = [i for i, x in enumerate(range(5))]
# Revealed type: `@Todo`
reveal_type(d)

f = [j for j in range(5)]
# Revealed type: `@Todo`
reveal_type(f)