Better list comprehension

Nobody has claimed this yet.

Sometimes writing `sum apply(L, a -> f(a))` gets too tedious, so we have defined syntax like `sum(L, a -> f(a))`. However, doing this to every similar function (e.g. `directSum`) is also tedious.

Can we allow something like `{ f : L }` to mean the same thing as `apply(L, f)`, so that we can write something like this, for instance?
```m2
N = directSum { a -> M(a) : 0..5 }
```
I think the precedence for `:` and `->` might need to be adjusted.

Alternatively, a flipped for loop like this might be nice:
```m2
N = directSum list M(a) for a from 0 to 5
```

Start by examining the existing syntax for sum, apply, directSum, and the precedence of `:` and `->`; no source file or test is named in the issue. The work is done when an agreed comprehension form is implemented and the proposed directSum example parses and behaves as intended.

Written by the indexing model from the issue text.