Lack of blanks makes source look confusing

```
constructor of LValue
interface constructor()

creates
uint x := 0
uint y := 2

invariants
y == 2

behaviour f of LValue
interface f()

storage
x => 0y => 2
```
This works all the way to `prove`, but `x => 0y => 2` itself looks a bit confusing to me.
Maybe we should force separators?

Start by examining the syntax handling for storage entries and the path exercised by `prove`, using the example in this issue as a reproduction. Determine how separators between `x => 0` and `y => 2` should be represented or enforced. Done means the source is unambiguous and the example still reaches `prove` successfully.

Written by the indexing model from the issue text.