`fn associativity(op: AssocOp) -> Associativity`

## Expand description

Returns the associativity/fixity of an operator. The difference with
`AssocOp::fixity`

is that an operator can be both left and right associative
(such as `+`

: `a + b + c == (a + b) + c == a + (b + c)`

.

Chained `as`

and explicit `:`

type coercion never need inner parenthesis so
they are considered
associative.