Add AST metadata about assoc operator location

[jonatanklosko]

Once we parse

%{:key1 => 1, :key2 => 2}

the entries become [{:key1, 1}, {:key2, 2}] in the AST and there is no information about =>. This adds a assoc: [line: ..., column: ...] to the key AST nodes.

cc @mhanberg

[josevalim]

💚 💙 💜 💛 ❤️